SCIENTISTIFIC INVESTIGATATION OF THE BRAIN-BASIS OF SIGN LANGUAGE

Scientistific Investigatation of the brain-basis of sign language

Scientistific Investigatation of the brain-basis of sign language

Since the early discoveries of the language-brain relationship (Broca, 1856; Wernicke, 1874; Geschwind, 1965; Goodglass, 1993) there has been a long debate about which cortical and subcortical areas support language processes. The classical brain lesion-behaviour approaches suggest the following interpretation: patients with left hemisphere posterior (temporoparietal) lesions including Wernicke's area speak fluently and produce at least parts of a sentence with seemingly correct syntactic structures. However, their speech and their comprehension reflect limitations to process content words such as nouns and verbs (e.g. Berndt et al., 1997). The underlying deficit in these patients has been defined either as a deficit in the semantic representation of the mental lexicon (Zurif et al., 1974) or as a deficit `in accessing and operating on semantic properties of the lexicon' (Milberg and Blumstein, 1981, p. 381; Blumstein et al., 1982).

The brain lesion-behaviour approach has been complemented by neuroimaging studies, such as PET and functional MRI, which provide further evidence that the superior temporal gyrus and the supramarginal gyrus support lexical processes during auditory language comprehension (Petersen et al., 1989; Zatorre et al., 1996). These types of studies have led to a more detailed functional description of the frontal cortices. It appears that the Brodmann areas (BA) 45/47/46 are involved in semantic memory (Kapur et al., 1994), whereas BA 44 is active during phonologically sequencing (Buckner et al., 1995) as well as during grammatical processing (Stromswold et al., 1996). Rephrasing these latter findings it could be argued that BA 44 supports the application of phonotactic and of syntactic rules or procedures.

Although these approaches have advanced our knowledge concerning the particular subparts of the brain involved in language processing, they do not suffice to describe the interplay of different functional subsystems in time. The measurement of event-related brain potentials (ERPs), whose temporal resolution is in the millisecond domain, attempts to study the temporal course and interplay between different neurofunctional subsystems involved in language processing (Kutas and Hillyard, 1983; Neville et al., 1991; Friederici et al., 1993.

ERPs are small voltage oscillations that are measured at the scalp surface and are time-locked to the processing of external events. They involve a sequence of deflections (i.e. components) that reflect stages of information processing in the brain. Language-related ERP components with different temporal and spatial characteristics of brain activity have been identified (for an overview, see Kutas and Van Petten, 1988). Three different ERP components have been reported to correlate with different aspects of language processing during comprehension. Semantic processes are reflected in the so-called N400 component, a centroparietally distributed negativity that shows up about 400 ms after the onset of a word, which is not expected given the prior semantic context (e.g. Kutas and Hillyard, 1983). Studies that utilized intracranial electrodes recordings suggest the anterior part of the inferior temporal lobe as a possible generator for the scalp recorded N400 (Smith et ...