The goal of the last set of semantics and brain readings was to determine whether language-specific semantic deficits occur, and if so, whether they are associated with posterior temporal lobe regions that David and I have proposed to be involved in "lexical-interface" processes. The readings have provided relatively clear answers to these two questions: Yes and no, respectively.
First, some background. There is plenty of evidence supporting the view that word-level comprehension deficits in aphasia are predominantly semantic in nature. For example, such patients more often make semantic than phonemic errors in auditory word-to-picture matching tasks (see Baker et al. 1981, Neuropsychologia, 19:1-15; Gainotti et al., 1982, Acta Neurol. Scandinav., 66:652-65). Such deficits are loosely associated with posterior temporal regions in that Wernicke's aphasics present with such semantic deficits (Baker et al.) and Wernicke's aphasia is associated with temporal lobe lesions.
But are these deficits modality specific? That is, are they restricted to the audio-verbal modality, or are they more general? The evidence demonstrates that both patterns exist in aphasics with single-word comprehension deficits.
Of the papers we read this week, Chertkow et al. (1997: Brain and Language, 58: 203-232) provide the strongest evidence. They tested a group of 16 aphasics of various diagnostic categories (Global, Broca, Wernicke) using a range of tasks. Visual perception tasks were used to rule out visual perception as a source of their problems (all were in the normal range). Half of the subjects -- all of the Wernicke and Global aphasics, and none of the Broca patients -- were impaired in auditory word-to-picture matching when the picture choice array comprised a set of semantically related items. The same pattern held in another comprehension test that required subjects to answer forced choice conceptual knowledge questions about heard words (e.g., lemon: is it used with coffee or tea?). A force-choice picture version of this latter task was also administered (picture of lemon: subject must indicate whether it goes with picture of coffee or tea). Five of the 8 patients with single word impairments were equally impaired on the non-verbal version of this task, but 3 patients (including both of the Wernicke aphasics) improved to normal performance levels with the non-verbal task. Conclusion: In some patients with single word comprehension deficits, the semantic impairment is language-specific, on other patients, it is more general.
The paper by Goodglass et al. (1997, Brain and Language, 56:138-58) used a very different paradigm, concept similarity judgments, to assess modality specific effects. They found that making concept similarity judgments across modalities (hear: skirt, see: jacket, decide: same category or not) was harder for aphasics (measured in terms of RTs) than making the same judgments in an all visual format. Crucially, the reverse held for control subjects: auditory-visual pairs were more quickly judged than visual-visual pairs. Presumably, the cross modality difficulty in aphasia stems from trouble accessing semantic representations from auditory-verbal input.
So it seems that language-specific word-level semantic deficits do occur in aphasia, even if it is also the case that some aphasics seem to have deficits that extend beyond the linguistic system. This is what we'd expect. However, where are the lesions associated with the language-specific vs. the non-specific deficits? I'll discuss that in the next post.
Post a Comment