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The neurology of syntax: Language use without Broca's area

Abstract

A new view of the functional role of left anterior cortex in language use is proposed. The experimental record indicates that most human linguistic abilities are not localized in this region. In particular, most of syntax (long thought to be there) is not located in Broca's area and its vicinity (operculum, insula and subjacent white matter). This cerebral region, implicated in Broca's aphasia, does have a role in syntactic processing, but a highly specific one: it is neural home to receptive mechanisms involved in the computation of the relation between transformationally moved phrasal constituents and their extraction sites (in line with the Trace-Deletion Hypothesis). It is also involved in the construction of higher parts of the syntactic tree in speech production. By contrast, basic combinatorial capacities necessary for language processing - e.g., structure building operations, lexical insertion - are not supported by the neural tissue of this cerebral region, nor is lexical or combinatorial semantics.

The dense body of empirical evidence supporting this restrictive view comes mainly from several angles on lesion studies of syntax in agrammatic Broca's aphasia. Five empirical arguments are presented: experiments in sentence comprehension; cross-linguistic considerations (where aphasia findings from several language types are pooled together and scrutinized comparatively); grammaticality and plausibility judgments; real-time processing of complex sentences; and rehabilitation. Also discussed are recent results from functional neuroimaging, and from structured observations on speech production of Broca's aphasics.

Syntactic abilities, nonetheless, are distinct from other cognitive skills, and represented entirely and exclusively in the left cerebral hemisphere. Although more widespread in the left hemisphere than previously thought, they are clearly distinct from other human combinatorial and intellectual abilities. The neurological record (based on functional imaging, split-brain and right-hemisphere damaged patients, as well as patients suffering from a breakdown of mathematical skills) indicates that language is a distinct, modularly organized neurological entity. Combinatorial aspects of the language faculty reside in the human left cerebral hemisphere, but only the transformational component (or algorithms that implement it in use) is located in and around Broca's area.

Keywords

agrammatism, aphasia, Broca's area, cerebral localization, dyscalculia, functional neuroanatomy, grammatical transformation, modularity, neuroimaging, syntax, trace-deletion.

Yosef Grodzinsky of the Psychology Department, Tel Aviv University, and of the Aphasia Research Center, Department of Neurology, Boston University School of Medicine, has been investigating brain/language relations for many years. He has published numerous articles in the leading journals in linguistics, psycholinguistics, and cognitive neuroscience. His book, "Theoretical perspectives on language deficits", was published by MIT Press (1990). His current research (carried out through tests of brain-damaged patients, and fMRI) focuses on the neurology of combinatorial skills. Specifically, he is conducting a comparative study of the neural representation of mathematical and syntactic abilities.

0. Getting more precise

Advances in neuroimaging technology have increased our knowledge of the neuroanatomy of higher functions of the central nervous system: it is now possible to get a glimpse at our brain while it is in action. Yet this progress would not be possible without a greater understanding of the knowledge base and operations that underlie complex behavior. Parallel to the increasing precision of the technology, some progress has been made in our understanding of the cognitive architecture that underlies certain behavioral domains. In the study of brain/language relations, theoretical developments in linguistics have gone hand in hand with imaging, making a joint contribution to behavioral neurology: several decades into the study of language and the brain from a linguistic angle, there is now a relatively dense body of facts that can be seriously evaluated. This target article reviews central results, and use them to motivate some novel conclusions regarding the representation of language in the human cerebral cortex. The discussion will revolve around the choice of unit of behavioral analysis, and its theoretical import. An outlook on language that derives from current linguistic theory can draw a new and more precise picture of language and the brain.

The old Connectionist school - led by Broca, Wernicke, and Lichtheim (cf. Lichtheim, 1885, for a comprehensive exposition), and revived in our time by the late Norman Geschwind (e.g., 1970, 1979) - fortified belief in the existence of cerebral language centers. As clinicians, they mostly emphasized the patients' communicative skills, viewing language as a collection of activities, practiced in the service of communication: speaking, listening, reading, writing, naming, repetition, etc. Their characterization of the language centers derived from this intuitive theory, as for each activity they posited a cerebral center. The resulting theory of localization uses these activities as building blocks and takes them as the essence of human linguistic capacity. This view, based mostly on aphasia studies, is still held, especially in clinical circles, as illustrated by current clinical manuals and textbooks (e.g., Bradley, Daroff, Fenichel, & Marsden, 1996, p. 37). The leading neurology textbook in the United States explicitly espouses the teachings of the great neurologists of the 19^th century, depicting the perisylvian region as the location of language, and providing an internal division as follows (analytic categories of behavior are bolded):

1. Cerebral Loci for Syntax

1.1. The neuroanatomy of syntax: Lesion- and

imaging-based approaches

The movement to redefine the language centers has had its effect in certain circles: the neurolinguistic localizing schema of language perception may not have permeated the clinical literature, yet it is currently accepted in cognitive neuroscience. On this model, syntax is represented in the part of left anterior cortex that receives its blood supply by the superior distribution of the left middle cerebral artery, (i.e., Broca's area and its vicinity as indicated above) while semantics and the lexicon are posterior, located temporo-parietally around the Sylvian fissure (Alexander, Naeser, & Palumbo, 1990; Damasio & Damasio, 1989; Zurif, 1995a). Based on an ever growing experimental record, it is now widely accepted that the speech production problem in Broca's aphasia implicates syntactic mechanisms of some sort (cf. Goodglass, 1976, 1993; Grodzinsky, 1984a; Marshall, 1986; Damasio, 1992; Zurif, 1995a); it is also believed that in comprehension, there is a deficit to receptive mechanisms of grammatical analysis (Damasio, 1992; Goodglass, 1993; Grodzinsky, 1990; Zurif, 1995a). Wernicke's aphasia, by contrast (following a posterior lesion in and around Wernicke's area), is believed to involve the lexical and interpretive components of the language faculty (Goodglass & Kaplan, 1983; Damasio & Damasio, 1992; Zurif, 1995a). Crucially, although other pathological signs are found in the overall description of many, if not most, language disorders, they are not part of the model for brain/language relations, either because they come from nonlinguistic domains (e.g., anosognosia, non-fluency, or dysarthria), or because they are not pathognomonic of any particular syndrome.

This neurolinguistic model was formed on the basis of studies of pathology, which related neuroanatomy to linguistic function by correlating impaired behavior with morphological lesion data (PM, CT or MRI). With the advent of functional imaging, evidence for the same distinctions has been sought from normal language processing. Initial findings seem to corroborate the basic approach: studies of functional imaging (PET and fMRI) have detected involvement of similar regions in syntactic processing (Bookheimer et al., 1993; Mazoyer et al., 1993; Stromswold et al., 1996; Just et al., 1996; Bavelier et al., 1997); electrophysiological studies (ERP) that chart electrical activity during the performance of syntactic tasks have likewise been consistent with this picture (Neville et al., 1991; Kluender & Kutas, 1993; Münte, Heinze & Mangun, 1993; Friederici, 1995). The movement to revise the picture from one of activity-based cerebral centers to centers representing different levels of linguistic analysis, then, has appeared to be on the right track, and has thus become the prevailing view in neuroscience.

1.2. Contradictory results from aphasia

A careful examination of the experimental evidence, which includes the more recent results, leads to conclusions that are much less definite. The data even appear contradictory at times: Wernicke's aphasics have some disturbances in syntactic comprehension (Zurif and Caramazza, 1976; Schwartz, Li, Saffran & Pate, 1987; Shapiro et al., 1993; Grodzinsky & Finkel, 1998), whereas Broca's patients, while failing certain tasks that probe receptive syntactic abilities (Goodglass, 1968; Caramazza and Zurif, 1976), have shown success in others (Linebarger, Schwartz, & Saffran, 1983). Taken at face value, these findings cast serious doubts on the model, in which Broca's area (but not Wernicke's) supports receptive syntactic mechanisms. Yet, given certain provisos, the neurological model castill be maintained. Wernicke's patients' failures in syntactic comprehension are rather inconsistent and varied (Zurif & Caramazza, 1976; Shapiro et al., 1993; Zurif, 1995a), and are by and large ignored. In the case of Broca's aphasia, moves have been made to reconcile empirical contradictions: syntactic abilities have been broken down into tasks, and it has been claimed that "syntactic comprehension is compromised" (Martin et al.,1989), and that "grammaticality judgment is intact" (Linebarger, Schwartz, & Saffran, 1983). Common to such analyses is a rather "holistic" approach to the functional deficit, details of cerebral localization being of more concern than linguistic questions. Gross distinctions between form and meaning seem sufficient, and hence, less attention is paid to more detailed structural properties of linguistic stimuli. As a result, the neurological model of language has continued to prevail.

Yet it is quite possible that the apparent inconsistencies in the results discussed above are due to the fact that many analyses lump together complex grammatical systems without distinguishing among syntactic types that are used in experiments. Seeming experimental discrepancies may thus be reconciled upon examining the structural properties of experimental stimuli. If true, this possibility could lead to a new view of the functional role of the language areas. In particular, Broca's (and perhaps Wernicke's) aphasia may become a selective deficit to receptive grammatical (i.e., syntactic) mechanisms, affecting only subsystems of the syntax (in addition to overt problems in speech production). In this view, inconsistencies in experimental results are only apparent, and disappear once the right linguistic distinctions are made.

It is hard to overestimate the implications of these conclusions (if they are valid) to the neurological study of language: data on a partial syntactic deficit not only lead to a refinement of our view of brain/language relations; they also call for an experimental linguistic approach, involving extensive and systematic use of large varieties of sentence types as test materials. Aphasia studies, which enable tests of the most fine-grained aspects of language and their neural representation, should play a central role in this kind of research program. A review of the current experimental record, and the conclusions it leads to, is thus in order. I will argue that the move from activity-based descriptions of brain/language relations to task- and linguistic-level-oriented ones is insufficient. The evidence suggests that the main language areas of the brain follow the particulars of linguistic theory. The precise functional neuroanatomy of these regions must therefore be more specific linguistically. A review of the current state of the evidence (described, of necessity, in linguistic terms) is thus in order.

2. A syntactic approach to Broca's area

2.1. Language Comprehension without Broca's area: The centrality of syntactic movement

Broca's aphasia is best known for the non-fluent, "telegraphic" speech associated with it. The comprehension problem in this syndrome is less noticeable, and harder to detect. This is probably why Broca's aphasia had initially been thought to be only a speech production problem (with obvious consequences for the diagnostic schema); not until the 1970s, when controlled experiments on comprehension began to be conducted, did a deficit in this modality become apparent. Indeed, the standard diagnostic batteries, e.g., Goodglass & Kaplan (1983) have not even acknowledged a receptive disorder in Broca's aphasia. These authors merely noted that at the early stages, Broca's aphasics "may be confused by more complex spoken messages" (p. 55). Yet, when experiments began to be conducted, they revealed - much to the surprise of investigators - a disruption in syntactic comprehension. An experiment that is taken as a landmark (Caramazza & Zurif, 1976) compared "semantically irreversible" and "reversible" object relative clauses, such as the ball that the boy is kicking is red, and the cat that the dog is chasing is black. While Broca's aphasics were successful in comprehending the former, using semantic cues to get around their deficit, they failed on the latter, indicating that they were unable to use the relevant syntax to get at the correct interpretation. This result clearly did not fit the old Connectionist model that distinguished production and comprehension anatomically, locating the former anteriorly in Broca's area, and the latter in and around Wernicke's area.

This surprising finding, and others that ensued, triggered a theoretical attempt to put the newly discovered receptive problem on a par with its expressive counterpart: as the speech production deficit was already thought to disrupt syntax, it was initially claimed that Broca's aphasics also suffered "asyntactic comprehension" (Zurif & Caramazza, 1976; Caramazza & Zurif, 1976), and that their deficit was parallel in both domains, "overarching" both production and comprehension (cf. Bradley, Garrett & Zurif, 1980; Zurif, 1980 for elaboration).

This claim shaped the debate in the early 1980s. The need to come up with more precise statements, and make distinctions among linguistic types, was becoming apparent, and thus initial attempts were made to capture all aspects of the grammatical aberration in one descriptive statement that would, moreover, span all modalities. The belief in Broca's area and its surrounding tissue as the sole locus of syntax led to a hope that patterns of impairment and sparing in speaking, listening, reading and writing would all fall under the same generalization. Thus, labor was invested in obtaining such a generalization - a unified deficit analysis of comprehension and production in agrammatism (Zurif, 1980; Zurif & Grodzinsky, 1983; Grodzinsky, 1984a,b).

However, it quickly turned out that this outlook had been overly optimistic. The pattern of selective impairment was more intricate than this account could allow, and comprehension had to be set apart from production. New experimental results were coming in, creating a rather dense body of data that was drawing a new picture, in which the comprehension deficit seemed more restricted than previously supposed: on testing, Broca's aphasics showed near-normal abilities in comprehension and grammaticality judgment on many syntactic constructions and hence did not appear to have "asyntactic comprehension". There was a disruption, but it was restricted to certain aspects of syntax. It was becoming clear, then, that a distinction between different levels of linguistic analysis would not suffice, and that distinctions within syntax were needed to account for the comprehension deficit, just as they were in speech production. Much of syntax, then, was intact in comprehension, as became evident through experiments that mostly required interpretation (and as will be shown later, the production deficit also turned out to be more selective, though in a very different manner). The one clear exception, which had actually stood out since the beginning of the experimental investigations in the late 1960s, was transformational movement in the syntax, as indicated by marked comprehension deficiencies on structures derived by such operations (cf. Goodglass, 1968; Caramazza & Zurif, 1976; Schwartz, Saffran & Marin, 1980; Ansell & Flowers, 1982 for early results). These basic results have since been fortified by massive evidence, coming from different laboratories which use diverse experimental techniques. For almost all these patients, anatomical and pathologic data are availab: common to all is a positive diagnosis as Broca's aphasics on a standardized test batteries (i.e., BDAE, WAB for English, AAT for German and Dutch), and a focal lesion to the left cerebral hemisphere, caused in the majority of cases by occlusion of the left Middle Cerebral Artery. And the repeated finding is clear: patients that are diagnosed as Broca's aphasics do suffer a receptive disorder of syntax, yet a highly restricted one: There is clear evidence for near-normal performance in moother domains of syntax, coupled with sharp failures on structures containing transformational operations.

We will now examine the experimental record in detail, for it underscores the centrality of grammatical transformations in the comprehension deficit of Broca's aphasia, and as a consequence, delineates the functional role of Broca's area in language rather precisely. First, in comprehension, Broca's aphasics can construct basic syntactic trees (phrase structure) for simple sentences that do not contain intra-sentential dependency relations, such as actives (cf. for instance, Grodzinsky, Pinango, Zurif & Drai, in press, for a review); they are also near normal in detecting violations of phrase structure rules (Linebarger, Schwartz & Saffran, 1983; Grodzinsky & Finkel, 1998). Second, the patients seem to have no impairment to their lexicon in comprehension, namely, the part of the lexicon that interfaces with sentence grammar is intact. This is demonstrated by their ability to detect violations of subcategorization (Linebarger et al., 1983) and argument structure (Grodzinsky & Finkel, 1998), and by the normal time-course of their lexical processing when argument structure is at issue (Shapiro & Levin, 1990; Shapiro et al., 1993). Third, when required to carry out tasks that involve thematic (theta-)assignment, they are successful. The theta-part of the grammar is concerned with thematic roles that a predicate assigns its arguments, and the manner by which they are linked to positions in the sentence where these arguments are realized. Simply put, each position (i.e., Subject, Object) is associated in the lexical entry of a predicate with a theta -role (Agent, Theme, Goal, Source, and Experiencer), from which the semantics of this sentential position can be recovered (cf. Haegeman, 1991, for a review). In this domain, we know that Broca's aphasics have intact abilities. They know the theta-roles of predicates, and are able to assign them directly to positions. This is evident from their normal performance in comprehension tasks that involve direct theta-role assignment in simple structures such as active sentences (cf. Grodzinsky, 1990 for an exposition). They also never violate constraints on the thematic structure of sentences (i.e., the theta-criterion) when they construct syntactic representations (Lapointe, 1985). Fourth, these patients can even compute some (but not all) intra-sentential dependencies and are able to detect violations of rules that govern them. One piece of evidence in this respect is their ability to detect case (Nominative, Accusative, Dative, and the like) on noun phrases. Mechanisms for case assignment are conditions that license the presence of NPs in their base positions. In certain languages, these conditions have overt consequences, through case markers that are realized phonetically. Broca's aphasics have virtually intact abilities in this domain, as can be seen from their performance in case marking languages, e.g., Serbo-Croat (Lukatela, Crain & Shankweiler, 1988; Crain, Shankweiler, Gorell & Tuller, 1989). They can also represent other complex intra-sentential dependencies that are defined over tree-structures. Specifically, they can handle the formal aspects of binding relations, which are the conditions that define the anaphoric relations between pronouns and reflexives, and their antecedents (cf. Grodzinsky, Wexler, Chien, Marakovitz & Solomon, 1993; See also Crain & Shankweiler, 1985). Some relations among pronouns, reflexives and their antecedents are impaired, yet these have to do with discourse-related aspects of pronominal reference, not with syntax (cf. Grodzinsky & Reinhart, 1993).

In sharp contrast with the above evidence for intact receptive abilities, we see a rather severe deficit in the comprehension of constructions derived by transformational movement, as indicated by the following familiar patterns of results, taken from dozens of experiments that investigated aphasics' interpretive abilities. In these experiments, the patient typically listens to a ("semantically reversible") sentence that contains two argument NPs and is required either to select one picture among several (including a critical foil that depicts a reversal of the actors), or to make a truth-value judgment regarding the match between the sentence and a single picture (in which the roles are either correct or reversed, see Grodzinsky, 1990, for a review). This task is believed to probe the subjects' ability to assign theta-roles to positions where the manipulated variable is syntactic structure. Several tokens (10-20) are usually presented for each sentence type, allowing the evaluation of the patients' performance relative to chance, or guessing:

Construction type                                        Performance level

(1) a. The girl pushed the boy                              above chance

b. The girl who pushed the boy was tall            above chance

c. Show me the girl who pushed the boy           above chance

d. It is the girl who pushed the boy                     above chance

e. The boy was interested in the girl                  above chance

f. The woman was uninspired by the man        above chance

(2) a. The boy was pushed by the girl                 chance

b. The Boy who the girl pushed was tall             chance

c. Show me the boy who the girl pushed            chance

d. It is the boy who the girl pushed                       chance

e. The woman was unmasked by the man         chance

This list is compiled from a large number of studies: for the basic active/passive contrast (1a)/(2a) there are results taken from 17 different studies, with at least 42 patients, all diagnosed positively as Broca's aphasics on standard tests; for the subject/object relatives (1b-c)/(2b-c) there are at least four studies of 16 patients (see Grodzinsky et al., in press, for a detailed review); three studies on 7 patients have documented the contrast between subject and object cleft sentences (1d)/(2d) (Ansell & Flowers, 1982; Caplan & Futter, 1986; Hickok & Avrutin, 1996). The contrast between (1) and (2) manifests not only in performance levels - it is also structural. As we shall see, the sentences in (2) are derived by a transformation in a way that hampers the patient's comprehension, whereas those in (1) do not. Note also that passive constructions feature in both (1) and (2), as emphasized by italics. We return to this issue later. At any rate, the contrast (1e-f)/(2e) was obtained in one study of 5 Broca's aphasic patients (Grodzinsky, Pierce & Marakovitz, 1991, see end of section 2.3.1 below). Critically, the contrast between (1) and (2) cannot be explained by appeal to familiarity, or frequency, since this type of account would need to show that (1f), for instance, is less frequent than (2a), or that (1c) is more frequent than (2c). Such demonstrations are unimaginable.

It thus appears that most aspects of syntax, whether pertaining to basic relations or to the more intricate dependencies, is intact in the comprehension of Broca's aphasics, with one salient exception: Syntactic movement - grammatical transformations.

2.2. The Trace-Deletion Hypothesis

A grammatical transformation is a complex operation over structural representations of sentences in natural language. Very roughly, it involves the copying of a constituent to another position in a sentence and the substitution of the material in the original position by a trace - a phonetically silent, yet syntactically active category that plays several important functions, two of which are relevant here: First, it is through the link between the trace and its antecedent that theta -roles (which are always assigned to canonical positions) are transmitted indirectly. Uniformity considerations force verbs to assign their assoctheta -roles in the same direction, regardless of the ordering of constituents around them. Hence, when a constituent is moved, it is through the link between it and the trace that its theta-role is transmitted. Thus in (3a) the role of Theme (recipient of action) is assigned by the verb like to its right hand constituent (=t), as it would be in an active sentence; the mutual index shared by the trace and its antecedent which man is the vehicle for theta -transmission. As a result, the latter becomes Theme:

(3) a. [which man]_i did Mary like t_i

      b. *[which man]_i did Mary spread the rumor that she liked t_i

What performances follow from trace-deletion? What linguistic behaviors would a traceless language user exhibit? In interpretation, it would imply problems in movement-derived constructions and the preservation of all else; in judgment, it would predict that violations of grammaticality would go undetected if traces are crucially involved in the determination of grammaticality; and in processing, it would predict real-time problems in linking antecedents to the positions they vacated. All three predictions are borne out. What remains is to examine the distribution of traces and inquire whether trace-deletion cuts the experimental pie correctly. The demonstration will also tie this deficit to the anterior language areas, by linking this functional impairment to Broca's aphasia.

A first hint comes from an examination of the patients' performance on passive constructions. In (1)-(2), predicates with passive morphology feature (italicized) in both cells, as they generate both guessing behavior (2a,e), and near-normal behavior (1e,f). This finding correlates perfectly with trace-deletion, since passive morphology does not necessarily imply a syntactic movement transformation. Certain passives (known as "lexical" (1e,f)) are base-generated, whereas others ("verbal" (2a,e)) are derived by a transformation, hence their representations contain a trace (cf. Grodzinsky, Pierce & Marakovitz, 1991). Trace-deletion partitions these data correctly (movement <=> comprehension problem). Yet this discussion glosses over important particulars. As we will see, there are still constructions that do contain movement, but where the aphasics perform normally; there are also distinctions among types of erroneous performance that trace-deletion cannot handle. In short, a more detailed analysis of both the syntax and the experimental tasks is in order.

Consider, first of all, interpretive tasks that require theta -role assignment - the basic data set in (1)-(2). If traces mediate the transmission of theta-roles (such as Agent, Patient, Experiencer, etc.) to moved constituents, then the result of trace-deletion would be that moved NPs would lack a theta-role. This may provide a formal means of partitioning the data: impaired structures contain traces, whereas the rest do not. Yet mere partitioning is insufficient: while it points to the line dividing these structures that give the patients trouble from those that do not, the actual performance rates does not follow from just trace deletion: trace-deletion explains, perhaps, why a moved constituent does not have a theta-role, yet it does not imply chance performance on the passives or object relative clauses. On the above assumptions, the rest of the grammar is intact. This should give patients enough information to carry out a thematic assignment task and get around the deficit - they should be able to infer the missing theta -role from the rest of the available information (i.e., the theta -structure of the predicate, the fact that another NP is assigned a theta -role directly, and all other grammatical principles, cf. Grodzinsky, 1990, ch. 5; 1995a).

Another consideration that points to the inadequacy of trace-deletion is internal to the linguistic framework. Current approaches to syntax assume traces in virtually every structural representation. Even in simple active-declarative sentences there is movement of the subject from a (deep) position inside the verb-phrase into the overt position of subject, (cf. (4a), where the VP is inside bolded brackets). This is known as the VP-internal subject hypothesis (Kuroda, 1986; Kitagawa, 1986; Koopman & Sportiche, 1988): subjects are base-generated inside the VP, and are forced to move up the tree. If true, this hypothesis may have the consequence that even actives can receive a theta-role indirectly. If traces are deleted, then subjects of active sentences will have no theta -role, because the trace, under this hypothesis, may be crucial for theta -transmission. Yet such sentences pose no comprehension problems to the aphasics. Trace deletion, then, is an insufficient account of the data. Something must be done to remedy this.

Moved constituents lack a theta-role for aphasics, because of the deletion of the trace. Yet, every NP must have some role in interpretation. It was proposed that moved NPs are assigned a role by a non-linguistic, linear default strategy, which in the cases discussed assigns the Agent role to traceless clause-initial NPs (this is somewhat reminiscent of Bever's (1970) influential proposal, and similar to Jaeggli's (1986) proposal for the objects of by-phrases in derived nominals). The interaction of this strategy with the grammar results in compensation for certain structures, but confusion for the cases that are performed at chance. For example, consider a schematic representation of the verbal passive taken from (1a), and object relative clause (2b):

                        Normal assignment

                Agent                             Theme

(4) a. [The boy]_i [_VP t_i pushed [the girl]]                                         above chance

            Agent                             Theme

                    Agrammatic assignment

                    Normal assignment

            Theme                                                   Agent

      b. [The boy]_i was [_VP t'_i pushed t_i] by [the girl]                         chance

            Agent                                                     Agent

                 Agrammatic assignment

In (4a), the subject normally receives the role. It moves from the VP internal position, where it leaves a trace. The correct role is transmitted through the trace to the subject, which moves leftward. In Broca's aphasia, this transmission does not occur (as trace deletion precludes theta-transmission), yet the default strategy assigns the subject the Agent role, which happens to be correct, and normal performance follows. In the case of passive (4b), the oblique object (the NP argument of the by-phrase, the girl) gets the Agent role. Crucially, no syntactic movement is involved in this part of the representation, and hence, the agrammatic patient is able to assign it properly. By contrast, the subject of the passive, the boy, is movement-derived in two steps, hence two traces, both linked to the subject: first, it moves out of its base object position (t) and lands in the VP-internal subject position; second, it raises from the latter position [Spec, VP] to its final place [Spec, IP] (cf. Burton & Grimshaw, 1992 for discussion). The link between these positions is the channel for theta-role transmission. In agrammatic Broca's aphasia, both traces are deleted, hence no such channel is available, which means that the subject of a passive sentence receives no theta -role grammatically. At this stage, the strategy kicks in, assigning Agent to this NP. So we have a thematic representation with one Agent in the by-phrase (assigned grammatically) and one in the subject (assigned strategically). This situation does not allow for a unique determination of the Agent of the action, and guessing follows, resulting in chance-level performance. Next, compare the account for subject (5a) vs. object (5b) relatives:

In (5a), the VP-internal trace (t_i) is linked to a trace in subject position (t'_i), which is in turn linked to the head of the relative clause (the boy). The traces are deleted in Broca's aphasia, and the relative head lacks a theta-role. However, the strategy assigns it Agent, which is precisely what it would have received under normal circumstances. The cognitive strategy thus compensates fully for the deficit here (and in other cases in (1)), and the observed above-chance performance follows. In (5b), by contrast, the traces (t_i) and (t'_i) are also deleted and the strategy applies, yet here it gives an undesirable result: There are two Agents in the representation and the result is chance performance.

In sum, the performance rates of Broca's aphasics on the comprehension of all the above constructions is deduced by assuming trace deletion and a strategy. This combined claim has become known as the Trace-Deletion Hypothesis (TDH, whose current formulation, to be precise, is restricted to traces of constituent movement, cf. Grodzinsky, 1995a). The performance of the patients is deduced through either thematic competition, or compensation: The strategy always assigns an Agent label to clause-initial NPs. Thus, if a moved constituent is linked to a different theta -role normally (as in passive, object-gap relatives, object clefts and the like), this constituent now becomes Agent; and since there is another, grammatically assigned Agent in the thematic representation, the two Agents compete, thereby inducing chance performance by agrammatics. In cases where the moved NP was supposed to be Agent (such as subject-gap relatives, subject clefts, or actives under the VP-internal subjects hypothesis), this role is not assigned normally through the trace due to trace deletion, yet the strategy correctly compensates by assigning that NP the Agent role by default.

Direct evidence for the validity of this strategy comes from an experiment with "psychological" verbs (Grodzinsky, 1995b). When required to assign theta-roles to in sentences containing such verbs, 4 Broca's aphasics (all suffering focal lesions as a result of left Middle Cerebral Artery infarction) performed below chance (i.e., they reversed theta-roles systematically, pointing to the wrong picture most of the time) on passives of psychological predicates such as in (6) (even though they performed normally on their active counterparts):

2. Syntactic movement, and precisely that, is disrupted along the lines of the TDH:

            A. Traces are deleted from Broca's aphasics syntactic representations.

Comparative aphasiology must begin with a choice of dimension alwhich one compares. Languages, after all, vary along many dimensions (as described, for example, by parametric theories of grammatical variation). A reasonable place to start is in basic word-order. This is a property with several manifestations: languages may manifest SVO (Subject-Verb-Object) as their basic word-order, SOV, OSV, etc. Moreover, languages may have secondary orders (for example, an SOV language, in which an active sentence would have this order, may also have an OSV order for active sentences). The mapping from basic to secondary word-order is done, in many cases, through a grammatical transformation. This cross-linguistic variation is thus very pertinent to the description of Broca's aphasia, and to our understanding of the neurology of syntax: The application of the TDH to different structures may depend on basic word order. A comparison between languages with different basic orders - English, Japanese and Chinese - may thus provide clues regarding the cerebral organization of syntax.

2.4.1. Comprehension in Japanese Broca's aphasia

Hagiwara (1993) has conducted a series of experiments on the comprehension of Japanese speaking Broca's aphasics. Her impressive findings provide an unusual angle on the deficit and underscore its restrictive nature: They show how movement structure, not the active/passive distinction, determines agrammatic comprehension performance.

The linguistic description of basic Japanese word order has been the subject of debate. One initial puzzle comes from the two kinds of active sentences it has, featuring overt SOV and OSV orders (7a-b). Japanese was initially analyzed as a "non-configurational" language (Hale, 1983), but it is now widely accepted that the SOV order is the basic, "non-scrambled" one (8a), whereas OSV (7b) is secondary, derived by a transformation that moved the object across the subject (Saito & Hoji, 1983; Saito, 1985; see also Fukui, 1993; Miyagawa, 1997). The evidence for this assumption comes from a variety of directions but is mostly based on the behavior of anaphoric expressions when they are in the different positions, which leads to the conclusion that Hanako, the object in (7b), must c-command the Verb Phrase, hence it must have moved to adjoin to a higher projection than that of Taro, the subject:

(7) Active:

a. Non-scrambled (basic):

        Taro-ga Hanako-ni    nagutta                                         above chance

The scrambled case is derived by a movement transformation, hence its representation contains a trace. What would be the prediction of the TDH regarding agrammatic performance in Japanese? The configurations of the active sentences in (7) are given in (7'), together with the linked theta-roles for agrammatics:
 

(7') a. S         O           V                                                            above chance

(8) Passive:

a. Direct (derived):

        Taro_i-ga Hanako-ni t_i nagu-rare-ta                                     chance

          Okaasan-ga musuku-ni kaze-o hik-are-ta                             above chance

According to Hagiwara, the sentence in (8a) is a standard, "direct" passive, derived by a movement transformation. The representation is annotated, and contains a trace of movement which is co-indexed with its antecedent - the moved NP Taro (with its nominative case -ga). The by-phrase is Hanako-ni. Trace deletion should result in the subject Taro not being able to receive its theta -role through the chain of movement. It is thus subject to the Default Strategy that assigns it the Agent theta-role; the by-phrase Hanako-ni is intact, just as in the English case, hence Hanako receives the theta-role of Agent. The result is a thematic representation with two Agents, and hence chance performance is predicted. Broca's aphasics indeed perform at chance on this structure, as do their English counterparts. By contrast, the "indirect" passive in (8b) is not derived by movement (as Hagiwara argues, following Kubo (1990) and others). Although it has passive morphology on the verb (-are), this construction has several properties that lead to the conclusion that it is base-generated rather than transformationally derived. For example, the by-phrase in (8b) is not optional, whereas in a regular passive it is. Following the general prediction of the TDH (movement <=> comprehension problem), Broca's aphasics perform above chance on this structure, although it contains passive morphology, is more complex (having a tree with more nodes), more loaded semantically (having two predicates), and has more words than constructions that lead to chance performance, such as the "scrambled" active, or the "direct" passive.

The results from the comprehension abilities in Japanese Broca's aphasics provide a comparative perspective that demonstrates the stability of this syndrome across languages as well as the generality of the proposed account. Moreover, it underscores the claim that the deficit in Broca's aphasia is not related to passive morphology. The data cluster in a way that cuts across this factor: Broca's aphasics score above-chance on basic actives and indirect passives (with passive morphology), and at chance on derived (scrambled) actives and direct passives, as the following table shows:

Yi-Ching Su (1994) has investigated the comprehension of relative clauses in Chinese in 2 Chspeaking agrammatic Broca's aphasics. Of interest is the peculiar structure of Chinese relative clauses: while the overt basic word order is SVO (as seen in the bracketed relatives in (10)), heads of relative clauses, as well as complementizers (bolded in (10)), follow the relative, contrary to English (11); this clear contrast in phrasal geometry correlates, rather interestingly, with the performance of Broca's aphasic on relative clauses in the two languages - whereas object relatives (11b) are impaired in English, and subject relatives are preserved (11a), Chinese relative clauses give the exact opposite pattern (10):

(10) Chinese relative clauses

        a. Subject: [t_i zhuei gou] de_i mau_i hen da                                     chance

         b. Object: [The man]_i who_i [the woman pushed t_i] was tall       chance

The performance of Broca's aphasics is predicted only by the location of the trace, and its interaction with the strategy. Most importantly, the performance of aphasics cannot be specified through reference to construction type (i.e., subject- vs. object-relative clause) as the contrast between English and Chinese shows. This conclusion holds, despite debates regarding word order in Chinese (e.g., Huang, 1982; Travis, 1984). This is reminiscent of the discussion of Japanese, where the data could not be accounted for through a specification of constructions like active and passive, or a morphological distinction between them. The comparative examination has shown, then, that the determinants of the comprehension deficit in Broca's aphasia are phrasal geometry, the location of the trace, and the interaction of trace-deletion with the strategy. Finally, if the comprehension deficit is part of the diagnosis, parametric considerations (in this case - word order parameters) must be taken into account (see also Beretta, Hurford, Patterson & Pinango, 1996, for comprehension data on Spanish speaking Broca's aphasics that support this view). An "extensional" diagnosis of Broca's aphasia, i.e. one based on an enumeration of construction types on which the patients succeed and fail, is ruled out.

2.4. Combining perspectives: judgment and real-time

2.4.1. Results from grammaticality judgment

The results that have just been reviewed provide factual support for the TDH, yet they come from a single experimental perspective - that of direct tests of sentence comprehension. However, the TDH has a broader potential and can be coupled with other experimental methods. For example, traces are also involved in the operation of constraints on syntactic movement, as we saw in (3). The TDH predicts that violations of grammaticality, in which the trace is crucially involved, would go undetected by agrammatic aphasics, as the claim is that these traces are deleted from the representation, and hence cannot participate in the determination of the grammatical status of a string. Such a finding, if obtained, would provide strong evidence for the claim that the deficit manifested in trace-deletion is indeed representational, and does not follow from a deficit to the theta -module.

Schwartz, Linebarger, Saffran and Pate (1987) have obtained results that provide preliminary clues regarding this issue. They conducted a "plausibility judgment" study in which their patients were asked to make judgments about whether or not sentences made sense. Their design intersected syntactic movement with semantic plausibility, and with length: regarding the semantic factor, the patients were given semantically implausible sentences in which the source of the implausibility was either one of the arguments (12a) or two (12b); syntactically, there was an argument that was either in situ (12a), or moved transformationally (12c-d); lengthwise, the semantic violation was also nested in a "padded" sentence that contained many words (12e), but no movement. Naturally, each violation had a plausible counterpart:

(12) a. #The puppy dropped the little boy                                  success

(14) The priest enjoyed the drink¹ that the caterer was² serving t³ to the guests

This is precisely what on-line experiments on normal language users have discovered. Through different methods - Cross Modal Lexical Priming (CMLP) being a central one - reactivation of antecedents in the position of their traces has been shown for subject- and object-relatives (Swinney et al., 1982; Swinney & Nicol, 1989; Swinney & Osterhout, 1990; Swinney & Zurif, 1995), for passive (McDonald, 1989), and for other structures. The experiments take the drink as prime, and while the sentence unfolds auditorily, a target is flashed on a screen at points (1)-(3). The expected finding, then, is that if a target word, say, juice, is presented visually to subjects at points (1), (2) and (3) when they are listening to the sentence, and the subjects have to make a lexical decision on it, priming effects will be documented at (1) and (3), but not at (2).This is what is found: priming effects are obtained only in (1) and (3).

Now, consider the TDH and the expected real-time behavior of Broca's aphasics in CMLP. Deleted traces mean no reactivation at the trace. This means that only in point (1) would a priming effect be obtained. Decay would explain the lack of an effect in (2), and the correlate to trace-deletion would be a lack of priming in (3). Conducting such experiments is quite difficult, yet this is precisely the result of a series of carefully controlled studies of both subject and object relative clauses (Zurif et al., 1993; Swinney & Zurif, 1995). Importantly, Broca's aphasics do prime, even if not in a fully normal fashion (e.g., Swinney, Zurif & Nicol, 1989; Shapiro & Levine, 1990; Shapiro et al., 1993)). Yet, when faced with a task that involves priming within a movement-derived construction, they are seriously impaired. Finally, this failure is not characteristic of all aphasics, nor is it necessarily related to general comprehension skills: Wernicke's aphasics with posterior perisylvian lesions perform normally on this task, even though their comprehension abilities are severely compromised.

We have gone through various sorts of evidence, among which tests of real-time syntactic analysis. This review led to a new delineation of the deficit. But can this lead to an unambiguous statement regarding the underlying cause? An answer to this question would lead to an explicit theory of the function of the anterior language areas. The question, obviously, is whether or not this area supports a device dedicated to syntactic analysis of transformationally moved constituents, and if so, what this device is. One possibility is that the comprehension deficit follows from a general disruption to "working memory", not from a language mechanism. A recent PET finding (Jonides et al., 1997) indicates that in nonlinguistic memory tasks that require the subject to relate two non-adjacent members in a list, Broca's area gets activated in a secondary fashion. To some this may sound like proof that this area supports a memory cell, the disruption of which entails the syntactic deficits in this syndrome, yet such a conclusion is a bit hasty. Although it may turn out to be truin some future, "final" analysis, such a conclusion is not warranted on the basis of the available evidence. To argue for a disruption to a generalized memory resource, one must show that this resource makes contact with formal constraints on the inner workings of this memory store from which constraints on syntactic movement would follow. Moreover, we have seen that other real-time performances of Broca's aphasics show how intact their language processing device is. Lexical access and insertinto sentential positions - found intact by Shapiro and his colleagues - demonstrate directly how specific the impairment is. We may then move on, and consider other, language specific, interpretations, namely: Is Broca's area home for a processing device whose disruption precludes antecedent reactivation at the right time? Or is it, perhaps, a representational medium that makes silent categories disappear? It is not possible, at this point, to decide between these options, yet either way, we have delineated the deficit and have shown that any deficit analysis, hence any theory of the role of Broca's area, must have the consequences of the TDH.

Consider, first of all, the most linguistically detailed study available - Stromswold et al.'s PET investigation. They measured blood flow during visual exposure to sentences whose plausibility the subjects were requested to judge. The relevant conditions included several experimental variables: relative clauses that were either (plausible or implausible) right-branching subject gaps (15a-b), or (plausible or implausible) center-embedded object-gaps (15c-d):

(15) a. The biographer omitted the story that insulted the queen

2.6. Generality and counter-evidence

The degree of generality of the foregoing is appreciable: dozens of studies were reviewed here, including many neurologically intact subjects as well as brain-damaged patients, tested in different laboratories at different times, in different countries and languages, on a broad variety of sentence types. The tasks reviewed come from comprehension in several languages, grammaticality and plausibility judgment, and real-time processing. The results converge. Still, it is important to note that sstudies yield seemingly contradictory results. These divide in two: failures to replicate previous results, which have sometimes led to the claim that the syndrome of Broca's aphasia gives an altogether inconsistent picture (e.g., Martin et al., 1989; Druks & Marshall, 1995; Berndt et al., 1996); and those that document performances that run contrary to the TDH but accept basic premises. (e.g., Hickok, Zurif & Canseco-Gonzales, 1993; Hickok and Avrutin, 1995). Both types of presumed inconsistencies must be taken very seriously: an unstable syndrome (and certainly a non-existent one) is the wrong object of inquiry; likewise, a false hypothesis is, most likely, the wrong one to follow. It must be revised, perhaps even abandoned, when confronted with data that cannot be accounted for. Thus, an old debate has been revived in recent years, in which the coherence of the clinical categorization, as well as the validity of the have TDH been challenged (cf. Badecker and Caramazza, 1985; Miceli et al., 1989 for attacks on the syndrome-based conception; also Hickok et al., 1993; Lukatela, Shankweiler & Crain, 1995; Berndt, Mitchum & Wayland, 1997, for critiques of the TDH on the basis of new data). Many (if not most) objections have been explained away: in some cases patient selection was the cause of the inconsistency, as patients that were not Broca's aphasics were included in the experimental group; in others, there were problems with experimental procedures or design; in still others, the results may have been misanalyzed or misinterpreted (cf. Caplan, 1986; Zurif, Gardner & Brownell, 1989; Grodzinsky, 1991; Zurif, 1996).

Still, even after this type of cleanup, a certain amount of inter-subject variation persists. Grodzinsky, Pinango, Zurif and Drai (in press) have recently conducted a survey of comprehension scores of Broca's aphasics in two contrasts that pertain to the TDH: actives vs. passives and subject-gap vs. object-gap relative clauses. For both sentence types there are multiple studies: the review covers 17 different studies of active/passive, with 42 different patients, and 4 studies of subject/object relatives and clefts, with 17 different patients. All the comprehension experiments we analyzed had a binary choice design (containing the correct response and its thematic reversal). The expectation from the results of the active sentences was a pattern approaching 100% correct. For the passive case, chance performance was expected; thus, since patients are said to be guessing, the performance of each patient should be equated with a series of (unbiased) coin tosses, as such series are exactly the expression of chance behavior. As such series are known to distribute binomially, with a median of around 50% correct, we expect our patients to follow this distribution. Moreover, we expect a statistically significant difference between the performances on actives and passives. The analysis of the actual data reveals that this is the case: the actives cluster around the 100% mark; the passives distribute binomially with a mean and median of 55%; the two differ significantly from one another. Figure 1 shows the distribution of performances for actives (dashes) and passives (dashes and circles), where the latter is compared to a computer-generated model of an averaged binomial distribution (full line):

Figure 1: the distribution of performances for 42 Broca's aphasics (# of patients vs. % correct) for active (dashes) and passive (dashes and circles)sentences. Full line is a computer-generated simulation of a binomial distribution (adapted from Grodzinsky, Pinango, Zurif & Drai, in press).

2.7.1. Morphological correlates of error types

The focus of this paper is comprehension. Yet Broca's area, so the evidence suggests, subserves not only receptive mechanisms of language, but also, language production. In fact, the most salient feature of Broca's aphasia has always been effortful, non-fluent and telegraphic speech. Below, I review some recent results in the production of language in Broca's aphasia, because they emphasize 2 points: first, they show the difference between the deficits in expressive and receptive mechanisms of language; second, they again show the need for an abstract grammatical approach to language and the brain.

Consider, first of all, a salient cross-linguistic difference in the behavior of agrammatic Broca's aphasic speakers, and the way they reveal how linguistic distinctions are honored by the brain. The omission of functional elements in speech production has always been the hallmark of the diagnosis of agrammatism in Broca's aphasia (e.g., Marshall, 1986; Adams & Victor, 1993, p. 417; Goodglass, 1976, 1993). Yet it is well established that patients either omit or substitute inflectional elements, but only if the morphology of the ambient language permits such omissions, as the patients observe rules of lexical well-formedness, and never produce non-words or word-parts (Grodzinsky, 1984a, 1990): in both verbal and nominal inflected elements omissions of inflectional morphemes are observed if a bare stem is a real, licit, word (+zero morphology); otherwise (i.e., in words that are -zero morphology, where omission of inflection is illicit and results in non-words), substitution errors occur:

(19) a. English

            Uh, oh, I guess six month...my mother pass away.

       b. Hebrew

            tiylu                                                              anaxnu ba'ali ve-'ani

        c. Russian

            grustnaja malchik.       stol stoyit,               vot, stol stoyat,             stoyit

        d. Italian

            Cappucetto rossa andava

        e. Japanese

            inorimasu (correct: inorimasushita)

The same cerebral pathology manifests differently, depending on the morphology of the language spoken: in English and Japanese we observe the familiar pattern of omission of the inflectional morphology, whereas in the rest of the languages we see an inflectional element substituted for another, with a resulting grammatical aberration, but lexical structure is unimpaired. Zero-morphology is thus relevant: Broca's aphasia in a language where stems are legal words presents with omissions; otherwise, there are substitutions. Grammars (or sub-grammars) align themselves accordingly:

(20)	+zero morphology	-zero morphology
	English	Hebrew
	Japanese	Russian
		Italian
	omission	substitution

 

(21) Broca's aphasics tend to omit inflections if they speak a language

With a zero-inflectional morpheme; otherwise, they tend to substitute.

Since omission has always been a critical diagnostic sign of agrammatism in Broca's aphasia, clinicians cannot ignore the variation just presented if they are interested in a precise and universal diagnosis. They must accordingly enter the conditional statement in (21) into the clinical diagnosis of agrammatic Broca's aphasia to allow for variation in error type as a function of grammatical (morphological) properties of the language spoken. Similar to the cross-linguistic TDH, the description of speech production errors across languages leads to a parametric definition of the syndrome, one in which grammatical parameters (imported from a theory of possible grammatical variation) are embedded. We return to this issue below.

(23) Reading French aloud:

(24)                 Yesterday the boy walked;
                                /                               \

            Tense condition                                  Agreement condition

The results were remarkable: While agreement was normal, tense was severely impaired, even though the patients' perception of time, as well as comprehension of temporal adverbs, was shown to be intact. Tense errors were mostly substitutions of inflection (with no preferred "unmarked" form), observed in repetition (25), and in completion (26) tasks. In (27) a numerical representation of error rates is presented:

(25) Target: ha'anashim yixtevu mixtav la-bank

        Repeated: ha-anashim katvu mixtav la-bank

(26) Target: axshav ata holex. etmol `ata _______(expected: halaxta)

        Completed: axshav ata holex. etmol ata telex

(27)

Agreement Errors	Tense Errors
3.9% (5/127)	42.4% (62/146)

 

2.7.4. A restrictive, structure-dependent account: the Tree-Pruning Hypothesis

This rather rich cluster of cross-linguistic facts has led to a description of agrammatic speech production that is stated over trees, not elements. That is, unlike every previous statement, which looked at functional elements regardless of their position in the sentence, the currently available data lead to the view that agrammatic aphasic patients produce trees that are intact up to the Tense node and "pruned" from this node and up (Friedmann, 1994; Friedmann & Grodzinsky, 1997):

(28) Agrammatic phrase marker. Arch represents site of deficit.

Interestingly, this claim receives empirical support from yet another direction: there is a salient cross-linguistic difference in the production of verbs by Broca's aphasics. In English, the speech output of Broca's aphasics contains verbs that are bare stems, yet these are located in their proper position in the sentence - always after the subject. In verb-second (V2) languages (e.g., Dutch, German), however, where inflected verbs undergo movement, the situation is different. In these languages verbs start out in sentence-final position (SOV see Koster, 1975; den Besten, 1983, but see Zwart, 1993 for a different analysis), and must raise to pick up their tense features, and the result is SVO order. A non-finite verb in a main clause (for instance, in a clause that contains an inflected auxiliary), will remain in final position, and its finite counterpart will be in second position. In a patient whose syntactic tree is pruned, verbs will fail to raise, and the result will be as is observed in Dutch: in aphasic speech verbs in main clauses not only appear uninflected, but also, are in sentence final position, resulting in ungrammatical strings (Kolk & Heeschen, 1992; Bastiaanse & van Zonneveld, 1998; Friedmann, 1998). Dutch agrammatics make no errors on infinitives in subordinate clauses but have major difficulties with inflecting main verbs, which they mostly produce not only as infinitives, but, critically, in final position. Finally, it has been observed that verbs tend to be omitted relatively frequently from the speech output of English speaking Broca's aphasics (Berndt & Zingeser, 1990). As Friedmann (1998) shows, a problem in the lexical category "verb" is apparent only if this result is looked at in isolation. Yet when the broader context is examined, a clearer picture is revealed. In particular, when cross-linguistic patterns of verb omission are reviewed, as Friedmann has done for English and Dutch, it turns out that only tensed verbs are omitted (cf. also Bastiaanse & van Zonneveld, 1998), which is precisely what the Tree-Pruning Hypothesis predicts.

Six new observations need to be highlighted:

1. The agrammatic production deficit is very strongly and directly linked to grammatical variables (the Tense node and its configuration).

2. This deficit is more restricted than previously thought, encompassing functional elements above a certain node in the syntactic tree; the rest of the representation is intact.

3. The precise description of the deficit (hence the diagnosis of the syndrome) must be stated in an abstract terms that allow (as linguistic theory does) cross-linguistic grammatical variation (e.g., infinitives in sentence-final position in V2 languages).

4. The availability of the rest of the syntax to speech production in the absence of Broca's area and its vicinity means that these cortical regions can longer be viewed as housing syntax as a whole. Rather, major parts of the human syntactic capacity reside elsewhere.

5. The deficit to mechanisms of language production, while sharing important features with its comprehension counterpart, differs from it in important ways (Tree Pruning vs. TDH). This conclusion may run contrary to claims regarding a parallelism between comprehension and production (e.g., Zurif, 1980), but it does not entail a total lack of a comprehension deficit in Broca's aphasia (Miceli et al., 1983; Kolk & van Grunsven, 1985; see Zurif, 1996, Grodzinsky et al., in press for counter arguments). There is a comprehension deficit, but its description is different from the production problem. In the normal language processing device, mechanisms for the planning and construction of sentences must diverge at some point from those dedicated to the analysis of incoming strings. Nevertheless, it is most likely that both mechanisms connect to one grammatical resource; hence, to some extent at least, they may be located in adjacent cerebral areas.

6. The account is somewhat reminiscent of recent accounts of children's grammar. First, like children (Hyams, 1992; Poeppel & Wexler, 1993), agrammatics have at least some functional categories - those below the Tense node (TP). Second, children and Broca's aphasics' production seem to have the same problem in verb inflection. This similarity, however, is only apparent: both groups produce incorrectly inflected main verbs; yet, whereas the aphasics substitute tense inflection, children only use the nonfinite forms, but never substitute inflection (Wexler, 1994). Finally, children, like the aphasics, produce matrix clauses with nonfinite verbs which lack elements that belong to higher parts of the tree - wh-words, complementizers, subject pronouns, and auxiliaries. But there is a crucial difference between children and Broca's aphasics: Children are able to build these constructions (Rizzi, 1994), whereas the aphasics cannot project any higher than T.

3. Clinical issues: Diagnosis and remediation

3.1. Comparative Aphasiology and parametric diagnostic principles

The pathological manifestations of agrammatic Broca's aphasia as presented here are verclose not just to grammar, but also to grammatical variation, in that a complete clinical picture depends on the patient's language: in language production, omissions are observed in +zero-morphology languages, and substitutions otherwise; verbs appear in their correct position in most languages, but in verb-second (V2) languages, they are in a sentence-final position; in comprehension, errors occur in a fashion that depends on the interaction between syntactic movement and phrasal geometry, which is determined by word-order parameters. Variation between patients in both production and comprehension thus exists, because the grammatical parameters that interact with their deficit have different values in different language types. Importantly, variation in this syndromeis not wild but constrained and can be characterized precisely, as the cross-linguistic differences at issue are relatively well-defined. Despite all this, an odd situation is created, in which diagnostic principles for universal identification must be established for a disease that has more than one manifestation. Broca's aphasia is thus an unusual pathology, with varying, yet well-defined clinical signs that depend mostly on cerebral structure, but also on an environmental factor - the ambient language. A precise clinical characterization must be predicated on trace deletion (and tree pruning), which interact with parametrically characterized aspects of morphology and word-order. Clinical testing will accordingly abide by these two descriptions, yet will vary from one language to another: physicians and speech pathologists can no longer test, say, the comprehension of active vs. passive sentences in every language, but rather the comprehension of sentences with and without movement. Diagnosis, in other words, must rely on abstract principles which are instantiated differently in each language.

Yet this is something unheard of in medicine. The medical literature does not contain a syndrome which is defined parametrically. That is, there is no disease which is diagnosed by a cluster of abstractly characterized pathological signs, each having more than one possible concrete manifestations whose form is determined by properties of the individual patient, or his environment. Broca's aphasia, oddly enough, is such a disorder.

3.2. Recovery and Remediation

Once the functional deficit in Broca's aphasia is characterized more precisely, one can perhaps try to think about treatment. That this is at all possible is far from obvious: an understanding of the precise nature of the deficit by no means guarantees the success of therapy since, after all, the patients have lost a piece of their cortex. Still, it may lead to the invention of better methods, which might also be equipped with better evaluation and efficacy-assessment tools. A fascinating development in this direction has been the program set forth by Shapiro, Thompson, and their colleagues, who have investigated the course of recovery in Broca's aphasia along linguistic lines, in an attempt to devise novel methods for speech therapy. They have made some remarkable discoveries: First, they found that recovery proceeds along structural lines: when a syntactic construction reappears in speech, it is accompanied with its structural analogues (Thompson, Shapiro and Roberts, 1993). Second, they devised an experimental therapy for aphasics, in which they have succeeded in training patients on movement (Shapiro and Thompson, 1994; Thompson, Shapiro, Tait, Jacobs, and Schneider, 1996). In one recent study, for example, they trained their patients on one construction, and subsequently monitored their abilities on three others (Thompson, Shapiro, Ballard, Jacobs, Schneider & Tait, 1997). Teaching was controlled carefully, as was the assessment of the patients' abilities on the other structures (29)-(30):

(29) NP-movement a. Passive: The biker was lifted by the student

                                   b. Raising: The student seems to have lifted the biker

(30) Wh-movement a. Object-cleft: It is the student who the biker lifted

                                  b. Object-question: Who has the biker lifted?

Although still experimental, such results are important, as they show once more that neural tissue abides by fine structural constraints: if valid, this training program shows that the internal structure of the grammar. Specifically, aspects of syntactic movement determine not only the breakdown pattern subsequent to focal lesion, but also, the progress towards recovery through the aid of external stimulation. And, although many questions still remain open, for example, the relation between remediation of questions in speech production and the deficit as characterized by the Tree-Pruning Hypothesis (cf. Friedmann, 1998, for extensive discussion) this development seems very promising.

4. Tentative conclusions and implications

The empirical evidence amassed so far leads to some conclusions:

A. Lesions to Broca's area and its vicinity do not affect semantic abilities, nor do they disrupt basic syntactic abilities. Most notably, Broca's aphasics combine lexical meaning into propositions, create and analyze sentences of considerably complex structure, and are also able to synthesize and analyze words morphophonologically. It thus follows that most human linguistic abilities, including most syntax, are not localized in the anterior language areas - Broca's area and deeper white matter, operculum, and anterior insula.

B. Broca's aphasics do suffer important, albeit limited syntactic deficits: their ability to construct full-fledged tree structures in production is compromised, as is the link between traces and their antecedents in comprehension. Processes underlying these highly structured syntactic abilities, and only these, are located in the anterior language areas.

C. Mechanisms that underlie language production are at least partially distinct from the comprehension device. Parts of both are located in left anterior frontal cortex, separate, but perhaps equal, which is why a lesion there disrupts each partially, and differently. Thus, although Broca's aphasia affects both modalities, strong parallelism between the deficits is not maintained, suggesting anatomical proximity, but functional separation, between production and comprehension mechanisms.

D. Cerebral lesions provide a unique testing ground for linguistic claims. If language knowledge and use are taken to be biologically supported, then a theory of linguistic representation and use must be compatible with patterns of language breakdown. Indeed, there have been several attempts to harness neurolinguistic evidence in support of particular linguistic claims (Grodzinsky, 1984b; Grodzinsky, Pierce and Marakovitz, 1991; Grodzinsky et al., 1993; Beretta et al., 1996; Friedmann & Grodzinsky, 1997; Pinango, Zurif and Jackendoff, in preparation). The richness of our language organ, and the very fine patterns of breakdown subsequent to brain damage, are likely to yield further, ever finer results, provided that the search is guided by a well-articulated theory.

5. What language is not - a modular approach

We have shown that Broca's area and its vicinity are home to one central syntactic ability, and that other abilities are intact following lesions to this cerebral region, which might indicate that the rest of the grammar resides elsewhere. Where, then, do human syntactic abilities reside? Are they separate from other cognitive skills? Could grammatical transformations be the only neurological expression of a distinct language faculty? These questions are particularly acute in light of the belief that has come back into vogue, that humans do not possess a special "languaorgan" (cf. Seidenberg, 1997, for a recent example). The apparent lack of localization of certain basic combinatorial linguistic abilities may lead to the suspicion that they are distributed over the cerebral cortex. A survey of the neurological record shows that this is not the case. The cerebral localization of syntax as a whole is restricted to the left hemisphere. Moreover, a comparison between language and other cognitive deficits upholds what has become conventional wisdom among linguists: combinatorial aspects of language are distinct from "general cognition." Linguistic arguments and evidence to that effect have been given (cf. Chomsky, 1995a,b for some recent discussion). In the following sections, some neurological evidence will be reviewed. This will indicate that, unlike other comabilities, syntax, though less localized than previously believed, is localized in the left hemisphere, and is distinct from other, seemingly related, intellectual capacities.

5.1. Language in the right hemisphere

As a first pass we look at the non dominant hemisphere - the right for most humans. We can assert unequivocally: no combinatorial language abilities reside in the non-dominant cerebral hemisphere. We have seen one direct test of right hemisphere syntax that hardly detected any activity during exposure to certain syntactic types (Just et al., 1996; Bavelier et al., 1997). Deficit analyses lead to the same conclusion.

Two sources of pathological evidence exist: split-brain patients, and patients with damage to their right hemisphere. In the former case, it is by now agreed that "unlike other language functions, complex grammar skills are localized to only one hemisphere" (Lustep, Wessinger & Gazzaniga, 1995). This conclusion follows from failures of a disconnected non-dominant hemisphere (left in the patient studied) to understand either active or passive sentences. Although the cerebral organization of split-brain patients may sometime differ from normal functional localization owing to their past history (often beset with childhood seizures), the finding has been consistent: series of studies by these authors and by others have documented a number of additional failures of the right hemisphere to process syntax correctly (Baynes & Gazzaniga, 1988; Gazzaniga, Smylie, & Baynes, 1984; Baynes, Tramo & Gazzaniga, 1992).

Lesion studies of right hemisphere damaged patients have corroborated this time and again. Van Lancker & Kempler (1987) compared the performance of right- and left-hemisphere patients the in comprehension of idioms and familiar phrases, with their ability to comprehend novel sentences, which may touch on the syntactic impairment of aphasics. The comprehension of "familiar phrases" may require an ability to extract nonliteral meaning, but no combinatory capacity, as these phrases are presumably stored in the mental lexicon; novel sentences, by contrast, cannot be stored and their comprehension requires analytic mechanisms. Subjects were requested to match pictures to sentences such as he is turning over a new leaf, where the pictures related either to the literal or the metaphoric meaning of the expression, and to sentences like when the angry girl pushes, the happy boy swings, where the distractor picture contained reversed thematic roles. Although the precise syntactic details of the materials used are unfortunately not reported the comparison between right- and left-hemisphere damaged patients still resulted in an interaction: the right-hemisphere patients, worse than aphasics on the comprehension phrases with some metaphoric value, were much better than the aphasics in the comprehension of unfamiliar sentences, for which they had to use their syntactic knowledge. Similarly, Zaidel, Zaidel, Oxbury & Oxbury (1995), have shown that right-hemisphere damaged patients, but not their left-hemispheric counterparts, are near-normal in their perception of syntactic ambiguities. This finding is especially interesting given the presence of long-distance dependencies in the stimuli, such as the elephant is ready to lift, which the patients successfully detected.

A series of related studies focuses on the linguistic abilities of right-hemisphere damaged patients. Here the relevant evidence is less direct. Brownell and his colleagues (cf., Brownell, Carol, Rehak & Wingfield, 1992; Brownell, Pincus, Blum, Rehak & Winner, 1997; Joanette & Brownell, 1990) have long been looking at properties of the overall communicative skills of these patients (with an intact left hemisphere), who are deficient in a number of ways, notably in the capacity to integrate aspects of discourse, metaphor, and other communicative conventions. Notably, no results could be obtained had the patients not been attuned to complex syntax, because in most cases the tasks require understanding complex stories, and answering questions afterwards. Major syntactic deficits would preclude success in these experiments. However, right-hemisphere damaged patients are able to perform these tasks, demonstrating their combinatorial capacity in the language domain.

Thus the evidence is that this side of the brain has an important an role in communication, but makes no syntactic contribution to language use.

5.2. Language and mathematics: two distinct combinatorial capacities

So language is in the left hemisphere. Yet given that the neurological organization of combinatorial linguistic operations turns out to be more widespread than previously believed, suspicions immediately arise regarding its distinctness from other formal operations. That is to say, linguistic abilities may not be distinct, but rather, follow from a general capacity to form complex combinations. In particular, mathematics and language may be the same. In other words, if the approach embraced here is to have any merit, it must demonstrate the cerebral distinctness of language and mathematics.

These problems concerned several great European neurologists (Henschen, 1920; Hecaen, Angelergues & Houillier, 1961; see Boller & Grafman, 1983, Kahn & Whitaker, 1991, for historical reviews), to whom it was clear that deficits in mathematical abilities must be set apart from language, memory or attentional problems. They realized that intact mathematical abilities can easily be masked by deficits to cognitive systems that are normally recruited for mathematical tasks. Thus in the early days, Hecaen et al. proposed, in their pioneering work, a distinction between primary (independent) and secondary (consequent) acalculia. The former - our current object of inquiry - is a varied disturbance: one kind is an "impaired spatial organization of numbers" (Levin, Goldstein, & Spiers, 1993) in which patients tend to misalign digits while carrying out basic arithmetical operations; another is "anarithmetria" - an inherent inability to carry out calculation (Benson & Weir, 1972). Critically, Hecaen et al. showed that it is a neurological entity in its own right, thus paving the way for serious discussion and investigation.

The current record appears unequivocal: both the clinical and experimental evidence point to the functional independence and neurological distinctness of mathematical and linguistic capacities. Ideally, we would like to observe language impairment with the retention of the ability to carry out complex mathematical operations, and vice versa. Unfortunately, brain damage of any type, even in patients who had been skilled mathematicians, makes it very difficult, if not impossible, to carry out cognitive tasks of high complexity. But although a direct result is hard to produce, we do have, at this time, a fairly rich array of evidence for both the functional and the neuroanatomical independence of linguistic and mathematical combinatorial abilities from studies of arithmetical skills. Two points can be made:

a. Neuroanatomical loci of mathematical skills, though reasonably spread, all appear to be retro-rolandic, and probably bi-hemispheric. That is, apart from some exceptional cases, only lesions in parieto-occipital regions (mostly in the left hemisphere but sometimes on the right), and certain temporal regions, can about primary acalculia (see Levin et al., 1993; Boller & Grafman, 1983; Kahn & Whitaker, 1991).

b. Anterior aphasia is functionally dissociated from primary acalculia (and dyscalculia); likewise, primary acalculia is dissociated from anterior aphasia or from damage to combinatorial linguistic skills (Levin et al., 1993; Rosselli & Ardila, 1989; Grafman, Passafiume, Faglioni & Boller, 1982).

Two of the more salient experimental results supporting these conclusions are reviewed below. Grafman et al. (1982) studied over 100 brain-damaged patients with cerebral lesions in various loci, whom they asked to carry out arithmetic tasks involving the 4 basic operations, with increasing difficulty (up to problems like 835+98279; 60100-4712; 308x73; 8694:69). They then the linguistic abilities of these patients on the Token Test (De Renzi & faglioni, 1967), tested them for constructional apraxia, and evaluated their intellectual abilities with the Raven Progressive Matrices Test. They found that, when age and educational level were controlled, patients with left posterior lesions performed significantly worse than all other groups on the arithmetical tests, whereas there was no significant difference between right hemisphere damaged patients, controls, and left anterior patients. Only the left posterior patients had mathematical problems that could not be attributed to linguistic, attentional, or other neuropsychological deficiencies.

It is critical to note that the scores of the anterior Broca's aphasics (n=30), while significantly better than that of the posteriors, was not significantly different from that of controls. So, although we have no further details of these patients, based on the fact that most of them (n=22) suffered a vascular accident, we can suppose that at least the majority were of the Broca's variety. Broca's aphasia, then, does not cooccur with acalculia (see also Dahmen, Hartje, Bussing & Strum, 1982, for similar conclusions).

Next, consider a study by Rosselli and Ardila (1989). They studied over 60 brain-damaged patients, carefully divided into several clinical categories. The patients carried out a large array of tasks involving numbers: reading, writing to dictation and related tasks, mental computing of an orally presented problem in arithmetic (e.g., 55+38; 93-13; 13x12; 150/30), solving simple and complex problems in writing (up to 689+437; 421-277; 212x37=24; 818/356), reading arithmetical symbols, and counting. Broca's aphasics (diagnosed by the Boston Diagnostic Aphasia Exam, Goodglass & Kaplan, 1983) made many errors on tasks requiring linguistic abilities (tests 31.1-31.2), in most instances more than other types of aphasics, but their error rate on tests of complex arithmetic (tests 31.3-31.4) was the lowest of all the aphasics, as shown in the table in (31), adapted from Rosselli and Ardila:

 

(31)	patient type | test  (% error)	1) reading	2) writing	3) mental	4) complex  written
	Broca	28.8	33.3	55.0	45.6
	Conduction	33.3	31.1	68.7	56.8
	Wernicke	20.0	17.7	66.2	68.0
	Anomia	13.3	13.3	75.0	72.0

 

5.3. Broca's area and general intellectual capacities

A non-modular view of language would attempt to make linguistic principles follow from theories of some other cognitive domain, of which general intelligence is a prime candidate. To show modularity, by contrast, is to show the distinctness of intelligence from combinatorial linguistic ability. An ideal neurological demonstration would show that the distribution of intelligence (measured by an IQ test of some sort, or some approximation thereof) in the aphasic population is identical with that in the general population. We are not likely to obtain data of this type, as large-scale IQ scores of aphasic patients are neither available, nor easily collected. Comparisons between the pre- and post-onset IQ measures of an aphasic group of patients (with some other brain-damaged controls) have not been done either. Direct evidence bearing on the question of language and intelligence, then, is not forthcoming.

There do exist, however, several studies that provide indirect demonstrations that intellectual capacity is truly distinct from language. Through different methods, they underscore the same point: most aphasias - Broca's, Conduction and Anomia (but not Wernicke's) - and intelligence are independent. This was first demonstrated by Kertesz & McCabe (1975), who compared aphasia type with intellectual ability of over a hundred aphasic patients. General intelligence was measured by the Raven Colored Progressive Matrices test - a standardized measure that contains non verbal measures of general intellectual skills, and is correlated with verbal IQ. Aphasia was diagnosed by the Western Aphasia Battery - a test that scores patients on fluency, repetition, comprehension, and naming, and divides them into the usual clinical categories. The mean scores of each clinical category were then compared to a neurologically intact control group. The result was that for Broca's (n=27), Conduction (n=11), and Anomic (n=40) aphasics, the scores on the intelligence test were not different from those of the normal controls; whereas the other clinical categories (Global, Wernicke's and Transcortical) were significantly lower. Similarly, Bailey, Powell and Clark (1981), who correlated scores on the Raven Matrices with a measure of severity of aphasia in a longitudinal study of over fifty aphasics (of unknown clinical category), found that while the linguistic abilities of the patients improved rather significantly over a 9-month period, their score on the intelligence test remained unchanged.

A different angle is found in Smith (1980), who constructed a task involving some kind of "non-verbal reasoning," in which the subjects were supposed to grasp relations such as "greater than" through a nonverbal demonstration of the experimenter on a set of wooden rods, and then carry them over, by way of analogy, to other domains (e.g., weight), where they were expected to solve a problem. Four of these patients seem to have been Broca's aphasics, and their performance was quite good, despite their language deficiency.

Finally, a comparison between IQ and auditory comprehension is a group of 98 aphasics (18 of which were Broca's), was done by Borod, Carper and Goodglass (1982). The Broca's aphasics were the only ones to be well above the group mean in their scores on the IQ test, producing the highesscores, whereas the Wernicke's and Global aphasics were the lowest, (the latter being significantly below the whole group).

Each of these studies has its problems and deficiencies: the measures of language skills that were used are questionable; the diagnostics may have been imperfect; some relied on dubious tests of intellectual capacity; doubts can probably be cast on at least some of the statistical tests. Still, the fact remains that no matter how aspects of language and intellect are assessed, the same result is repeatedly obtained: language deficits and intelligence in Broca's aphasia are distinct and independent; Broca's aphasics, although linguistically inferior to neurologically intact controls, are not different from them intellectually. This is altrue those suffering from of other aphasias, namely Conduction aphasia and Anomia (which are consequences of damage to distinct cerebral loci - the arcuate fasciculus and the angular gyrus, respectively). Moreover, sparing of intellectual skills does not seem to occur in all aphasias: Whatever one might think of the nature of the deficit in Wernicke's and Global aphasia, they certainly do not represent an exclusive failure of grammatical devices, and they involve (at the very least) some lexical semantic, and probably other disruptions to knowledge. Indeed, intellectual capacities are negatively affected in these syndromes.

6. Coda: Broca's legacy and the role of the language areas

Paul Broca, founder of modern neuropsychology, had three central ideas regarding the cerebral organization of cognitive functions. Recast in modern terminology, he said that (a) there is a one-to-one relation between neural substrate and behavioral function (at least for language) (b) language is distinct from other cognitive capacities; and (c) language resides in the left cerebral hemisphere, not its homolog on the right. Broca's legacy - modularity, neural representation, and lateralization - is here to stay: After nearly one hundred and forty years, and thousands of theoretical, clinical and experimental articles later, we can conclude rather confidently that all three properties Broca ascribed to brain/language relations are true: there is a distinct and dedicated "language organ" in the human left cerebral hemisphere.

To find the language loci and characterize them precisely, we must continue to re-"redefine" the language centers. Linguistic theory is the best tool currently available for this job. Once it is used, we discover that Broca's area is more specialized than previously supposed: it handles only intra-sentential dependency relations. This endeavor has another important potential benefit: it may help us to discover natural classes within our grammatical system, and to test the biological feasibility of grammatical theories. When we discover a pattern of grammatical impairment and sparing, it may reveal something about the internal structure of the grammar. Such arguments have been made in several domains (e.g., Grodzinsky et al., 1991; Grodzinsky et al., 1993, Hickok & Avrutin, 1995). Hopefully, more work will lead to further discoveries.

We may thus conclude that Broca's success was accompanied by one major error: he looked at communicative activities, ascribing no neural or cognitive value to the structure of the linguistic signal. When this mistake is corrected, his legacy can be preserved, and in fact, enriched significantly. At the same time we get a new, more precise, and deeper picture of the cerebral representation of linguistic functions: on this view, language is a distinct mental and neural faculty, with an inherent structure comprising a rich knowledge base and a processing device that implements it; this faculty resides in the left hemisphere. Yet, although Broca's area (and its surrounding left anterior neural tissue) is highly specialized and important, language is mostly not there.