Electrophysiological insights into language deficits

The human capacity for communication by means of a natural language is undoubtedly one of the most complex products of evolution. Most people seem to speak and listen effortlessly. And, yet, both psycholinguistic and neuropsychological research attest to the diverse set of representations that are accessed or constructed anew during ordinary language comprehension and production. Moreover, these access and construction processes are executed with amazing speed. For instance, adults can readily produce up to 15 speech sounds per second (Levelt, 1989) and can, on average, recognize a word in conversation (i.e. in context) within 200 ms (Marslen-Wilson, 1989). Thus, during both speaking and listening, the average adult manages quickly and unconsciously to gain access to the phonological, morphological, syntactic and semantic information that is appropriate for a particular lexical entry (i.e. word) from a database of between 40 000 and 70 000 words (Nagy and Herman, 1987). Clearly, the high speed with which word information is accessed imposes very strong constraints on the viability of any theory of the representational structure of the mental lexicon and the mechanism(s) that provide(s) access to it. Lexical access, however, is but one operation among the many that are essential for effective communication via language. Understanding speech is not simply a matter of accessing word forms. Once the acoustic stream has been segmented into units that access existing word forms, the language processor is faced with the task of combining the syntactic (e.g. word class and for verbs, argument structure) and semantic information associated with each word form in a way that is consonant with the speaker's intent. That is, recently accessed syntactic and semantic information have to be integrated with the representation of the 'meaning' of the utterance that is most consistent with all the available information up to that moment; this process of mapping the lexical information onto the intended meaning of the whole utterance is called lexical integration (Marslen-Wilson, 1984). It is likely that lexical integration is guided both by local constraints provided by recently accessed word forms and by the global constraints of the ongoing conversation or discourse; the latter must in turn be shaped by the overall context and pragmatics of the situation. In a variety of models that differ in other regards, the parser is credited with a central role in lexical integration. Specifically, it has been suggested that it is the parser that assigns each word in an incoming string its grammatical role in the sentence (cf. Frazier, 1987). Lexical integration also subsumes whatever mechanism assigns the different sentence constituents their thematic roles (e.g. agent, experiencer, theme), undeniably a necessary precursor to successful interpretation of a grammatical utterance. Whereas most models of language comprehension honor the need for a functional distinction between lexical access and lexical integration, considerable controversy still surrounds the question of whether and, if so, how these processes influence each other in real-time (cf. Frauenfelder and