The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

The mirror-neuron hypothesis also lends insight into autistic language difficulties. Mirror neurons are almost certainly involved when an infant first repeats a sound or word that she hears. It may require internal translation: the mapping of sound patterns onto corresponding motor patterns and vice versa. There are two ways such a system could be set up. First, as soon as the word is heard, a memory trace of the phonemes (speech sounds) is set up in the auditory cortex. The baby then tries various random utterances and, using error feedback from the memory trace, progressively refines the output to match memory. (We all do this when we internally hum a recently heard tune and then sing it out loud, progressively refining the output to match the internal humming.) Second, the networks for translating heard sounds into spoken words may have been innately specified through natural selection. In either case the net result would be a system of neurons with properties of the kind we ascribe to mirror neurons. If the child could, without delay and opportunity for feedback from rehearsal, repeat a phoneme cluster it has just heard for the first time, that would argue for a hardwired translational mechanism. Thus there is a variety of ways this unique mechanism could be set up. But whatever the mechanism, our results suggest that a flaw in its initial setup might cause the fundamental deficit in autism. Our empirical results with mu-wave suppression support this and also allow us to provide a unitary explanation for an array of seemingly unrelated symptoms.

Finally, although the mirror-neuron system evolved initially to create an internal model of other people’s actions and intentions, in humans it may have evolved further—turning inward to represent (or re-rep-resent) one’s own mind to itself. A theory of mind is not only useful for intuiting what is happening in the minds of friends, strangers, and enemies; but in the unique case of Homo sapiens, it may also have dramatically increased the insight we have into our own minds’ workings. This probably happened during the mental phase transition we underwent just a couple hundred millennia ago, and would have been the dawn of full-fledged self awareness. If the mirror-neuron system underlies theory of mind and if theory of mind in normal humans is super-charged by being applied inward, toward the self, this would explain why autistic individuals find social interaction and strong self-identification so difficult, and why so many autistic children have a hard time correctly using the pronouns “I” and “you” in conversation: They may lack a mature-enough mental self-representation to understand the distinction. This hypothesis would predict that even otherwise high-functioning autistics who can talk normally (highly verbal autistics are said to have Asperger syndrome, a subtype among autistic spectrum disorders) would have difficulty with such conceptual distinctions between words such as “self-esteem,” “pity,” “mercy,” “forgiveness,” and “embarrassment,” not to mention “self-pity,” which would make little sense without a full-fledged sense of self. Such predictions have never been tested on a systematic basis, but my student Laura Case is doing so. And we will return to these questions about self-representation and self-awareness, and derangements of these elusive faculties, in the last chapter.

This may be a good place to add three qualifying remarks. First, small groups of cells with mirror-neuron-like properties are found in many parts of the brain, and should really be thought of as parts of a large, interconnected circuit—a “mirror network,” if you will. Second, as I noted earlier, we must be careful not to attribute all puzzling aspects about the brain to mirror neurons. They don’t do everything! Nonetheless, they seem to have been key players in our transcendence of apehood, and they keep turning up in study after study of various mental functions that go far beyond our original “monkey see, monkey do” conception of them. Third, ascribing certain cognitive capacities to certain neurons (in this case, mirror neurons) or brain regions is only a beginning; we still need to understand how the neurons carry out their computations. However, understanding the anatomy can substantially guide the way and help reduce the complexity of the problem. In particular anatomical data can constrain our theoretical speculations and help eliminate many initially promising hypotheses. On the other hand, saying that “mental capacities emerge in a homogeneous network” gets you nowhere and flies in the face of empirical evidence of the exquisite anatomical specialization in the brain. Diffuse networks capable of learning exist in pigs and apes as well, but only humans are capable of language and self-reflection.