In a ‘‘virtual lesion’’ repetitive TMS (rTMS) study on speech perception, the TMS effects over premotor cortex were, if anything, a little stronger than the TMS effects over the auditory cortex (Meister et al., 2007). However, the effects were not reliably different, suggesting that both structures participated in the functional process, in contrast to GH’s suggestion that motor processes play a small, modulatory role in speech perception.Meister et al. found that TMS to premotor cortex resulted in a modest decline in performance in identifying synthesized CV syllables presented in noise in the context of a three-alternative forced choice paradigm. There has been no study that I'm aware of to show that such an effect is found when natural stimuli are used. The stimuli have to be degraded, i.e., partially ambiguous. Can we conclude that premotor cortex is playing an "essential role in speech perception" as the title suggests? No, we can only conclude that it is playing a modest role in the performance of an artificial task under degraded listing conditions. And we can't even tell what aspect of the task is being disrupted. It is possible that TMS is not interfering with the perception at all but rather interfering with the sensory-motor memory of which response button corresponds to which syllable. This one piece of evidence is held up to counter the array of studies that I cited showing that damage to the motor speech system, developmental failure of the motor speech system, complete biological lack of the capacity for a motor speech system, does not prevent speech perception. Where does the weight of the evidence leave us? The motor system plays a modest modulatory role if that. Why didn't STG stimulation cause a greater decline in performance? There is abundant evidence that speech perception is bilaterally mediated in the STG (Hickok & Poeppel, 2000, 2004, 2007).
Again, I find it counterproductive to focus on dichotomous models (‘‘it’s auditory,’’ ‘‘no, it’s motor’’). These models, although didactically useful, tend to provide a limited understanding of the functional processes at play. Indeed, consistent with the model in GH’s Figure 2D, the most successful recent computational models of action and perception disclose the intimate relationship between motor control and perception (Friston, Daunizeau, Kilner, & Kiebel, 2010; Friston, Mattout, & Kilner, 2011).I outlined four possible models, only two of which were dichotomous. I'm not denying that action and perception are intimately related. They are! But the functional relation is precisely the reverse to what the mirror neuron claim holds.
Eventually, we will have to get rid of these labels altogether, because they seem to get in the way of a better understanding of the phenomena under investigation.Call it what you like, it doesn't change the fact that systems in the posterior frontal lobe aren't necessary for speech perception, whereas bilateral systems in the superior temporal lobe are. As much as some folks would like the cortex to one big happy interacting neural network with no differentiation, the fact is that damage to different parts of the system have different effects. We have to deal with these facts. Returning to the facts, here's a quote from Meister et al.
The present results demonstrate that the involvement of the premotor cortex in perception is not merely epiphenomenal and suggest that sensory regions are not sufficient alone for human perception. p. 1695and a figure from Rogalsky et al. 2011 which shows comprehension, word discrimination, and syllable discrimination performance of two cases with lesions involving the human mirror system.
The recent follow up to Rogalsky et al. using a sample of 24 cases with Broca's area lesions confirms what was found in these two cases.
So, I've covered the response to my criticisms of mirror neuron theory by two of the most prominent and thoughtful defenders of the theory. Given the opportunity to present their strongest possible rebuttal to direct critiques in the Mirror Neuron Forum, both Gallese and Iacoboni failed to mount a viable defense of their model. This, of course, is my view. I'm sure they will disagree and again I invite them to post their own comments as guest entries on this blog. So far I have not heard a peep from either of them despite direct email invitations to participate.
Gallese, V., Gernsbacher, M., Heyes, C., Hickok, G., & Iacoboni, M. (2011). Mirror Neuron Forum Perspectives on Psychological Science, 6 (4), 369-407 DOI: 10.1177/1745691611413392
Hickok, G., & Poeppel, D. (2000). Towards a functional neuroanatomy of speech perception. Trends in Cognitive Sciences, 4, 131-138.
Hickok, G., & Poeppel, D. (2004). Dorsal and ventral streams: A framework for understanding aspects of the functional anatomy of language. Cognition, 92, 67-99.
Hickok, G., & Poeppel, D. (2007). The cortical organization of speech processing. Nature Reviews Neuroscience, 8(5), 393-402.
Meister, I. G., Wilson, S. M., Deblieck, C., Wu, A. D., & Iacoboni, M. (2007). The essential role of premotor cortex in speech perception. Curr Biol, 17(19), 1692-1696.
Rogalsky, C., Love, T., Driscoll, D., Anderson, S. W., & Hickok, G. (2011). Are mirror neurons the basis of speech perception? Evidence from five cases with damage to the purported human mirror system. Neurocase, 17(2), 178-187