Online research in philosophy

This commentary brings further support for the Trace Deletion Hypothesis (TDH) from a new study of OVS (Object-Verb-Subject) and OSV (Object-Subject-Verb) sentences in Hebrew, which are active constructions that involve object movement but no change in morphology. The comprehension of these constructions in Broca's aphasia is impaired, and the performance is at chance level, as predicted by the TDH.

Grodzinsky proposes that agrammatism leads to a “pruning” of the syntactic tree in speech production. For German, this assumption predicts that syntactic processes related to functional projections AgrP and CP should be impaired. An analysis of spontaneous-speech data from four Broca's aphasics with respect to subject-verb agreement and verb placement, however, indicates that phrase-structure representations in agrammatism are intact.

This commentary focuses on the larger implications of Grodzinsky's hypothesis. Although Grodzinsky argues persuasively that the syntactic comprehension deficits in Broca's aphasia involve mainly an inability to comprehend sentences requiring a transformational movement of phrasal constituents, his larger claim for a distinct and dedicated “language organ” in the left hemisphere is much less tenable.

Grodzinsky's localization claim can be questioned on empirical grounds. The Trace Deletion Hypothesis fails to account for a number of comprehension facts in Broca's aphasia and conduction aphasics show similar comprehension patterns. Frontoparietal systems are recruited during sentence comprehension only under conditions of increased processing load and/or attentional demands.

Many authors assume that Broca's area subserves the functions that are lost in patients with Broca's aphasia. This commentary attempts to clarify the relationship between Broca's area and Broca's aphasia and suggests that statements about the neurology of patients' specific language functions might be better supported by their individual structural neuroimaging data.

Grodzinsky associates Broca's area with three kinds of deficit, relating to articulation, comprehension (involving trace deletion), and production (involving “tree pruning”). Could these be special cases of one deficit? Evidence from research on language evolution suggests that they may all involve syllable structure or those aspects of syntax that evolved through exploiting the neural mechanisms underlying syllable structure.

A new view of the functional role of the 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 the 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 – for example, 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 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 are nonetheless distinct from other cognitive skills and are 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. Key Words: agrammatism; aphasia; Broca's area; cerebral localization; dyscalculia; functional neuroanatomy; grammatical transformation; modularity; neuroimaging; syntax; trace deletion.

Grodzinsky has argued that the traces deleted in Broca's aphasia are “phonetically silent but syntactically active” (sect. 2.). If we assume such traces to be visuospatial in nature, and adopt the term “overwriting” from the author's partial matching theory (1998), we can account for the errors made by Broca's aphasics in comprehending Grodzinsky's Examples (5a), (5b), and (6).

This commentary discusses the relation between Grodzinsky's target article and Friederici's (1995) model of syntactic processing. The two models can be made more compatible if it is assumed that people with Broca's aphasia have a problem in trace construction rather than trace deletion, and that the process of trace construction takes place during the second early syntactic substage of Friederici's model.

I wholeheartedly endorse Grodzinsky's program of attempting to tie the particular deficits observed in Broca's aphasics' comprehension and production to changes in their mentally represented model of grammar. At the level of detail, however, I see problems with two specific changes that Grodzinsky posits. One is a default Agent-assignment strategy in comprehension. The other is the hypothesis that production involves pruning all functional projections above Agreement Phrase.