Friday, May 2, 2008
Rizzolatti & Craighero (2004): Class discussion summary #6 (relation between hand and mouth)
There's an interesting discussion towards the end of RC's paper that covers evidence for links between hand and mouth gestures. This is relevant to the speculations on the mirror systems role in the evolution of speech. Links between hand and mouth are taken as evidence of a common neural foundation, and therefore a mechanism to transfer functions from one system (grasping) to the other (speech).
The interesting observation is that the act of grasping objects influences orofacial gestures. Gentilucci et al. (2001, J. Neurophysiol. 86:1685-99) showed that lip aperture during the articulation of syllable was larger if subjects were concurrently reaching for a larger compared to smaller object. Interesting. Similar evidence from other studies was discussed as well. But why the link. Drew Headley, a grad student in Norm Weinberger's lab here at UCI who is taking the "live" MN course, suggested a simple explanation: eating. Hand and mouth movements are associatively linked because we generally use our hands to stuff our mouths! This may even explain correlations in the size of the gesture. When we pop a grape in our mouths we have a smallish grip on the food and a smallish lip aperture to allow the grape in, whereas when we take a bite of an apple both our grip and our lip aperture are larger. So maybe the association between hand and mouth isn't necessarily related to a common, evolution-linked neural substrate. Maybe it's just because we eat with our hands.
Later on the paper a study is discussed in another context, where TMS-induced MEPs were recorded on the lip and hand (Watkins et al, 2003, Neuropsychologia, 41:989-94). Listening to speech enhanced the MEPs on the lip, but not the hand. This was used as evidence for the existence of an "echo-neuron" system. But it also demonstrates a clear dissociation between the hand and mouth using a now standard probe of mirror neuron activity (TMS-induced MEPs).
In short, it seems there is plenty of room for skepticism regarding the idea that "Hand/arm and speech gestures must be strictly linked and must, at least in part, share a common neural substrate" (p. 184).