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Colloquium on Neuroimaging of Human Brain Function
to be appropriate to the study of skilled reading of continuous text. This constraint led us to the hypothesis that the frontal area represent the meaning of the current word.
Our study represents a test of the hypothesis that the frontal area is involved in lexical semantics and the posterior area is involved in integration of words into propositions. The data from the single task condition fit well with the temporal and spatial locations defined by this hypothesis. Despite this support from the data, it may be surprising that frontal areas are involved in semantic processing because the classical lesion literature argues that semantic functions involve Wernicke’s area. Lesions in Wernicke’s area do produce a semantic aphasia in which sentences are uttered with fluent form but do not make sense. However, studies of single word processing, which prime a meaning and then have subjects respond to a target, have shown greater impairment or priming from frontal than from posterior lesions (23). Thus, the lesion data also provide some support for the involvement of frontal structures in lexical meaning and posterior structures in sentence processing.
Our data also suggest that the meaning of the lexical item is active by ≈200 ms after input. Psycholinguistic studies of the enhancement and suppression of appropriate and inappropriate meanings of ambiguous words suggest that the appropriate and inappropriate meanings are both active at ≈200 ms but that at 700 ms only the appropriate meaning remains active, suggesting a suppression of the inappropriate meaning by the context of the sentence (24). The time course of these behavioral studies fits quite well with what has been suggested by our ERP studies.
Another source of support for the relation of the frontal area to processing individual words and the posterior area for combining words is found in studies of verbal working memory (25). These studies suggest that frontal areas (e.g., areas 44 and 45) are involved in the rehearsal of items in working memory and that posterior areas close to Wernicke’s area are involved in the storage of verbal items. Such a specialization suggests that the sound and meaning of individual words are looked up and that the individual words are subject to rehearsal within the frontal systems. The posterior system has the capability of holding several of these words in an active state while their overall meaning is integrated. Also in support of this view is the finding that Wernicke’s area shows systematic increases in blood flow enhanced with the difficulty of processing a sentence (26).
The conjunction of a lexical category and a fit to a sentence frame is a very unusual task to perform. Subjects have to organize the two components, and it is quite effortful to carry out the instruction. Yet, as far as we can tell from our data, the anatomical areas that carry out the component computations remain the same as in the individual tasks. Thus, when subjects have to rely on an arbitrary ordering of the task components, they use the same anatomical areas as would normally be required by these components. However, the ERP data from the two conjunction conditions are quite different. This is rather remarkable because the two conditions involve exactly the same component computations. The results we have obtained are best explained by the view that the subjects use attention to amplify signals carrying out the selected computation and in this way establish a priority that allows one of the component computations to be started first. Such a mechanism would be consistent with the many studies showing that attention serves to increase blood flow and scalp electrical activity (4). Defining a target in terms of a conjunction of anatomical areas is a powerful method to use the subjects’ attention to test hypotheses about the function of brain areas. Our results fit well with the idea that frontal areas are most important for the classification of the input item and posterior areas serve mainly to integrate that word with the context arising from the sentence.
We are grateful to Y.G.Abdullaev for help in preparing Fig. 1. This paper was supported by a grant from the Office of Naval Research N00014–96–0273 and by the James S.McDonnell and Pew Memorial Trusts through a grant to the Center for the Cognitive Neuroscience of Attention.
1. Cabeza, R. & Nyberg, L. (1997) J. Cognit. Neurosci.9, 1–26.
2. Demb, J.B. & Gabrieli, G.D. (in press) in Converging Methods for Understanding Reading and Dyslexia, eds. Klein, R. & McMullen, P. (MIT Press, Cambridge, MA).
3. Posner, M.I., McCandliss, B.D., Abdullaev, Y.G. & Sereno, S. in Normal Reading and Dyslexia, ed. Everatt, J. (Routledge, London), in press.
4. Posner, M.I. & Raichle, M.E. (1994) Images of Mind (Scientific American Library, New York).