CHAPTER 5

Discussion

PAGE 9

The study of almost all cognitive tasks by PET and fMRI have produced complex networks of active brain areas. This is true even when subtraction is used to eliminate as many of the mental operations involved in the task as possible. One example of this complexity are the brain areas involved in comprehending the meanings of words and sentences. Two of the most prominent areas that have been related by many studies of comprehension are left frontal and left temporo-parietal areas.

Figure 6

Studies of the time course of these mental operations reveal some important constraints on the computations they perform. The frontal area is active at about 200 msec while the posterior becomes active much later. Because saccades have been shown to reflect access to lexical semantics and take place by 300 msec, only the frontal area is active in time to influence saccades in skilled reading. During skilled reading only one or two words are comprehended in any fixation. Thus the time course of comprehension of a word when it appears by itself must be well within 300 msec if the data from word reading is to be appropriate to the study of skilled reading of continuous text. This 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 while 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 since the classical lesion literature argues that semantic functions involve Wernicke's area. Lesions in Wernicke's area do produce a semantic aphasia in which sentence are uttered with fluent form but which 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 from frontal than from posterior lesions (21). Thus the lesion data also provide some support for the involvement of frontal structures in lexical meaning and posterior structures in sentence processing.
Figure 7

Our data also suggest that the meaning of the lexical item is active by about 200 msec 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 around 200 msec but that at 700 msec only the appropriate meaning remains active, suggesting a suppression of the inappropriate meaning by the context of the sentence (22). This time course of these behavioral studies fits quite well with what has been proposed 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 (23). These studies suggest that frontal areas (e.g. area 44, 45) are involved in the rehearsal of items in working memory, while posterior areas close to Wernicke's area are involved in the storage of verbal items. Such a specialization suggest that individual words look up their sound and meaning and 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 assessed. Also in support of this view is the finding that Wernicke area shows systematic increases in blood flow enhanced with the difficulty of processing a sentence (24).

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 upon 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 since 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 mechanisms would be consistent with the many studies showing 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.

p.9 Mon Nov 10 21:39:28 US/Pacific 1997