Thursday, May 17, 2007

Take a Good Hard Look at Your Mirror Neurons

Last month Alison Gopnik published an article on titled, "Cells that read minds: What the myth of mirror neurons gets wrong about the brain." Gopnik argued that mirror neurons have replaced left-brain/right-brain mythology as the neural basis for just about everything from human language, to social understanding, to art appreciation. It's not that the discovery of mirror neurons by Rizzolatti and colleagues is unimportant -- on the contrary, it is arguably one of the most significant findings in recent years -- it's that speculation about their function is completely out of control. I'm not sure it has quite reached the level of insanity that the left-brain/right-brain craze did (I recently came across a claim that left- vs. right-brain tendencies explains why men are "beer guzzling, TV-glued, and sex-driven"), but it wouldn't surprise me if mirror neuron-based personality, management, or learning-style self tests started popping up on the web. Of course, Gopnik could easily have gone even farther back in the history of neuroscience and drawn parallels between the current mirror neuron fad and 19th century phrenology, which, like left-/right-brain function, is another example of the over-application of legitimate scientific ideas. Hmmm... even the functions that are claimed to be supported by Gall's mental organs on the one hand, and the mirror neuron system on the other, are starting to sound alike: altruism, empathy, morality...

But these complex functions are way beyond my capacity to understand (not enough mirror neurons, I guess), so let's stick to language. Interestingly, as with phrenology, language was one of the first "applications" of the mirror neuron machinery. It was low-hanging fruit that came ripe with a rich (albeit ailing) cognitive grounding in the form of the Motor Theory of Speech Perception. Now in the case of phrenology, we know that the fundamental tenant of the theory -- that the cerebral cortex is functionally differentiated -- was ultimately proven correct by using language as a test case. Will the mirror neuron system parallel phrenology in this respect also? Not so much. Data from language research provides rather strong evidence against the mirror neuron theory of perception/understanding.

In the domain of language, the mirror neuron theory is basically this: speech gestures are understood in the listener via a mapping of heard speech onto speech production systems. This theory makes a very clear prediction: destruction of speech production systems should destroy the ability to understand speech. But Broca's aphasia disconfirms this prediction. In many cases of severe Broca's aphasia the entire convexity of the left frontal lobe is destroyed, along with the ability to produce speech. Yet these patients can understand speech quite readily at the lexical level. The same holds true in the visual/manual modality. Frontal lesions can severely impair sign language production in deaf signers while leaving sign comprehension rather nicely spared. If the "mirror system" forms the basis for understanding speech, Broca's aphasics should not be able to comprehend speech any better than they can produce it. Yet the evidence is clear: left frontal lobe, while critically involved in speech/sign production, is not critically involved speech/sign comprehension.

So the same clinical syndrome, Broca's aphasia, that confirmed the fundamental claim of phrenology, refutes the mirror neuron theory of speech/language understanding. Does this mean that the whole mirror neuron enterprise is misguided? Who knows. But if the most straightforward and cognitively grounded human application of mirror neuron function doesn't hold water, it certainly makes you wonder. In this context it is worth emphasizing that while mirror neurons are widely held to be the neural basis for imitation, and it is on this base that more elaborate functional speculations are built, the species in which mirror neurons were discovered, macaque, does not appear to have the capacity to imitate. Hmmm.

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