Copyright © 1999 Massachusetts Institute of Technology
Grammar refers to the syntactic structure of sentences that allows the meanings of words to be related to each other to form propositions. Linguistics has been concerned with the way humans' unconscious knowledge of this structure is represented. PSYCHOLINGUISTICS has been concerned with how this knowledge is used in speaking and comprehension. There is no way at present to investigate how the nervous system represents syntactic knowledge, but there are two approaches to the neural basis for syntactic processing. One has been the traditional deficit-lesion correlational approach in patients with brain lesions. The second is the observation of neurophysiological and metabolic activity associated with syntactic processing in normal subjects. Both approaches have made some progress, but there are many gaps in our scientific investigation of the question.
Deficit-lesion correlations are available for patients with disorders of both production and receptive processing of syntactic structures. With respect to the production of syntactic form, the speech of patients with a symptom known as agrammatism is characterized by short phrases with simple syntactic structures and omission of grammatical markers and function words. These patients tend to have lesions that include Broca's area (pars triangularis and opercularis of the left third frontal convolution), which has led some researchers to suggest that this region is responsible for syntactic planning in LANGUAGE PRODUCTION (Zurif 1982). Several studies have shown, however, that lesions in other brain areas can produce agrammatism, suggesting that other left hemisphere areas can be responsible for this function in some individuals (Vanier and Caplan 1990; Dronkers et al. 1994).
Syntactic processing can also be impaired in comprehension, as shown by patients' failure to understand sentences with more complex syntactic structures whose meaning cannot be simply inferred (e.g., The boy was pushed by the girl). More detailed studies of disorders of the time-course of syntactic processing in sentence comprehension have also been carried out (Tyler 1985; Swinney and Zurif 1995). The original studies of patients with syntactic comprehension disorders also focused on agrammatic Broca's aphasics (Caramazza and Zurif 1976). However, patients whose lesions lie outside Broca's area also often show impairments of syntactically-based sentence comprehension (Berndt, Mitchum, and Haendiges 1996; Caplan 1987; Caplan and Hildebrandt 1988; Caplan, Baker, and Dehaut 1985; Caplan, Hildebrandt, and Makris 1996; Tramo, Baynes, and Volpe 1988), and patients with agrammatism often show good syntactic comprehension (Berndt, Mitchum, and Haendiges 1996). This has led some researchers to suggest that a more distributed neural system in the left perisylvian cortex, of which Broca's area may be a specialized part, is responsible for this function (Mesulam 1990; Damasio and Damasio 1992). One study (Caplan, Hildebrandt, and Makris 1996) reported a small but clear impairment in syntactic processing in comprehension after right hemisphere strokes, suggesting some role of the nondominant hemisphere in this function.
Physiological and metabolic studies in normal subjects have also provided information about the brain regions involved in syntactic processing in comprehension. Event related potentials (ERPs) have shown components, such as the P600 or "syntactic positive shift" in the central parietal region and the "left anterior negativity," that may be associated with syntactic processing (Hagoort, Brown, and Groothusen 1993; Munte, Heinze, and Mangun 1993; Neville et al. 1991; Rosler et al. 1993). Recently, functional neuroimaging with POSITRON EMISSION TOMOGRAPHY (PET) and functional MAGNETIC RESONANCE IMAGING (fMRI) has been used to investigate the regional cerebral blood flow (rCBF) associated with sentence-level language processing. Using PET, Mazoyer et al. (1993) reported inconsistent rCBF increases associated with syntactic processing, but it may be that their experimental conditions did not differ in the minimal ways necessary to isolate the neural correlates of the various components of linguistic processing above the single-word level. Stromswold et al. (1996) reported an isolated increase in rCBF in part of Broca's area associated with syntactic processing. Using a slightly different experimental paradigm with fMRI, Just et al. (1996) reported an increase in rCBF in both Broca's area and a second language area -- Wernicke's area in the left first temporal gyrus -- as well as smaller increases in rCBF in the right hemisphere homologues of these structures. The Just et al. results are consistent with those of Caplan, Hildebrandt, and Makris (1996), but more research is needed to understand the differences across various studies.
In summary, the dominant perisylvian cortex is the region of the brain most involved in syntactic processing and production. Whether there is any further specialization within this region for these functions remains to be established.
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Copyright © 1999 Massachusetts Institute of Technology