In this context, recent studies of amyotrophic lateral sclerosis (also known as Lou Gehrig's disease in the U.S.) would seem to provide just the sort of evidence needed to prove the validity of the embodied cognition view. ALS is a disorder that severely affects motor neurons. If verb processing deficits can be documented in ALS, both in production and comprehension, this would be strong evidence favoring the embodied cognition viewpoint. Claims to this effect exist in the literature, but how solid are they?
One of the first systematic studies of language deficits in ALS, and the first to examine noun versus verb processing is Bak et al. (2001, Brain, 124:103-120). This is an excellent and thorough study that reports on six patients with a dementia/aphasia presentation of ALS. I'm no expert on ALS and I'm just starting to look at this literature, but this immediately raises questions in my mind. Are there cases of ALS with significant motor neuron degeneration but no aphasic symptoms? If so, this would seem to be good evidence against the embodied cognition view no matter what is found in the aphasic variant. Also, how much does the dementia component contribute to language deficits?
With these caveats in mind, let's consider the main findings from this paper. First, clear evidence of language deficits were found in the six ALS patients. Production was severely impaired as was comprehension as measured by the TROG (Test of Reception of Grammar). Of the patients who could perform a naming test, naming of actions was more impaired than naming of objects. A similar pattern was found on the noun versus verb comprehension test, where performance was 86% correct for nouns and 63% correct for verbs (chance = 50%).
Seems like a nice result for the embodied cognition view. This might be fairly compelling if the damage were restricted to motor and/or premotor areas and no higher-order cognitive deficits existed. But a consistent pathological finding was the involvement of BA 44 & 45. Area 44 is typically thought of as being a premotor area, but 45 seems to have much broader functions, being implicated in syntactic processing, response selection, etc. In addition, at least 5 of the 6 patients had prominent psychiatric and/or executive dysfunction, which not only implicate a broader network of brain system, but could also substantially impact test performance. If verb stimuli are more difficult to process than noun stimuli, the dissociation could simply result from a complexity effect rather than a disruption of action knowledge. A group of 20 Alzheimer patients were tested as a control group. They performed much better on the noun and verb comprehension test (>95%), and the noun-verb difference was "not significant" but numerically, AD patients were worse on verbs than nouns with a reported p-value of 0.075. As AD patients should show the reverse dissociation if anything, this may indicate a task difficulty effect.
In sum, variants of ALS can have language deficits that implicate frontal circuits and may affect verb processing to a greater extent than noun processing. The basis of this effect is unclear however, at least from this paper. It will be fun to dig a little deeper. Let me know if you come across a particularly compelling paper in this respect...
T. H. Bak, D. G. O'Donovan, J. H. Xuereb, S. Boniface, J. R. Hodges (2001). Selective impairment of verb processing associated with pathological changes in Brodmann areas 44 and 45 in the motor neurone disease-dementia-aphasia syndrome Brain, 124 (1), 103-120 DOI: 10.1093/brain/124.1.103