I'm still looking for compelling evidence that damage to the motor system affects verb processing. TMS data was not convincing, nor was ALS data (see previous posts). Now I'm looking at a lesion study by Neininger & Pulvermuller (2003, Word-category specific deficits after lesions in the right hemisphere. Neuropsychologia, 41:53-70), and I have to admit this is a reasonably impressive result -- but not exactly air tight.
Twelve patients with right frontal lobe damage and left hemiparesis were studied, as well as six patients with damage to right inferior temporal-occipital regions. None of the patients were aphasic, consistent with the fact that the lesions were in the right hemisphere. A group of control patients were also tested. Nouns with strong visual associations and verbs with strong action associations were presented visually and subjects were asked to make a lexical decision on each item. A third category of words was included that they called "bimodal nouns" -- nouns with both strong visual and motor associations.
The basic result was that frontal patients made significantly more lexical decision errors on verbs than on both categories of nouns, whereas the temporal-occipital patients made more errors on visual nouns than on verbs, and on visual nouns than bimodal nouns. Controls showed no differences. In reaction time data, there was an overall difference between bimodal nouns and action verbs, but no other effects. In sum, they got a nice double-dissociation in accuracy data, with frontal lesions impairing action verb judgments and temporal-occipital lesions impairing visual noun judgments. No such effect was seen in the RT data, however.
We could quibble about why the effect wasn't seen in RT data when such an effect would be expected (in fact both the frontal and the temp-occ groups were actually numerically slower on the verb than noun stimuli), or with the fact that three of the right frontal patients were ambidextrous, or with the fact that two of the temp-occ patients had sensory-motor defects. But let's take the result as valid with this set of patients.
There are a couple of problems. One is the varied location of the lesions, particularly in the frontal group. Eight of the 12 "frontal" patients had involvement of the temporal lobe, and seven had parietal lobe involvement. All had damage to the basal ganglia. No individual accuracy data was reported. To what extent can we attribute the pattern of errors to "motor cortex" let alone the frontal lobe? We can't. We can conclude that large middle cerebral artery infarcts in the right hemisphere can cause patients to make lexical decision errors on action verbs relative to nouns. This is interesting, but not exactly solid evidence for the view that motor systems support action word processing.
Another problem, which underscores problem number one, is that TMS data from Pulvermuller's group appears to refute the involvement of right motor cortex in action word processing. In the study highlighted in the previous blog entry, TMS to right motor cortex did not lead to any effect on lexical decision responses, unlike the reported effect in the left hemisphere. Apparently, something else is going on in the case of large MCA lesions.
B Neininger (2003). Word-category specific deficits after lesions in the right hemisphere Neuropsychologia, 41 (1), 53-70 DOI: 10.1016/S0028-3932(02)00126-4
I think their most compelling finding is that that temporo-occiptal patients were less impaired on "bimodal" nouns than visual nouns. Since a comparison between these two categories isn't confounded by imageability differences, what's the reason for the relative preservation of the nouns with strong action associations?
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