Monday, November 3, 2008

Ventral premotor cortex and action processing: Urgesi, et al.

Here is another pair of studies that a reviewer suggested I failed to discuss because they didn't support my pre-conceived hypothesis regarding mirror neurons. It's true that I didn't discuss them, but not because I cherry picked papers to discuss. I simply wasn't aware of these. After looking at them, I realized that they did not even test action understanding, so I could have justified leaving them out. Nonetheless, because they apparently are viewed a strong evidence for the link between the ventral premotor cortex and action processing, I included a discussion in my review. Here is a summary...

Urgesi et al. (2007a/2007b) used rTMS to study the effects of functional deactivation of ventral premotor cortex (vPMc) on visual discrimination of action-related pictures In both of these studies, subjects were asked to make two-choice, match-to-sample judgments: a picture of a body configuration was presented (the sample) followed by a mask (500msec), and then a picture of two body configurations; the subject was asked to indicate which of the two matched the sample.

First, as I mentioned above, it is important to notice that neither of these studies actually tested action understanding. That is, discrimination performance did not depend on understanding the meaning of the actions, and could be performed based on configural information alone.

Urgesi, Candidi et al. (2007) compared the effects of stimulation of vPMc with stimulation of a ventral temporal-occipital location (the extrastriate body area, EBA) during action discrimination (which action matches the sample?) versus form discrimination (which actor matches the sample, independent of action?).

For action judgments vPMc stimulation yielded longer reaction times than EBA stimulation, and the reverse held for form judgments, longer reaction times for EBA stimulation than vPMc stimulation. Stimulation had no effect on accuracy. In the other study (Urgesi, Calvo-Merino et al., 2007), subjects were asked to judge body configuration only, and an effect of accuracy was observed with vPMc stimulation associated with more errors on the configuration matching task than with EBA stimulation. Oddly, there were no reaction time effects.

So the two studies showed that interference stimulation to vPMc negatively affected performance on a body configuration delayed matched-to-sample task. Again, because these studies did not assess action understanding, they cannot speak to the question of whether the mirror system supports action understanding. However, they do suggest that processing of body configurations at least in the delayed match-to-sample task involves vPMc to some extent. Given that the tasks involved working memory, it seems possible that this region may support some sort of working memory for body configurations. This is interesting, but in my view is more consistent the idea that the "mirror system" is a sensory-motor integration system, not a semantic system. For example, there are many claims regarding the sensory-motor nature of working memory systems (Buchsbaum & D'Esposito, 2008; Hickok, Buchsbaum, Humphries, & Muftuler, 2003; Pa, Wilson, Pickell, Bellugi, & Hickok, in press; Postle, 2006; Ruchkin et al., 2003; Wilson, 2001).

Cosimo Urgesi, Matteo Candidi, Silvio Ionta, Salvatore M Aglioti (2006). Representation of body identity and body actions in extrastriate body area and ventral premotor cortex Nature Neuroscience, 10 (1), 30-31 DOI: 10.1038/nn1815

C. Urgesi, B. Calvo-Merino, P. Haggard, S. M. Aglioti (2007). Transcranial Magnetic Stimulation Reveals Two Cortical Pathways for Visual Body Processing Journal of Neuroscience, 27 (30), 8023-8030 DOI: 10.1523/JNEUROSCI.0789-07.2007

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