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

     (e) The MNS is not a dedicated set of hardwired neural circuits; it may be constructed through associative learning. For instance, every time you move your hand, there is activation of motor-command neurons, with simultaneous activation of visual neurons by the appearance of the moving hand. By Hebb’s rule, such repeated coactivations will eventually result in the visual appearance itself triggering these motor neurons, so that they become mirror neurons.

     I have two response to this criticism: First, even if the MNS is set up partially through learning, that wouldn’t diminish its importance. The question of how the system works is logically orthogonal to how it is set up (as already mentioned under point d above). Second, if this criticism were true, why wouldn’t all the motor-command neurons become mirror neurons through associative learning? Why only 20 percent? One way to settle this would be to see if there are touch mirror neurons for the back of your head that you have never seen. Since you don’t often touch the back of your head or see the back of it being touched, you aren’t likely to construct an internal mental model of the back of your head in order to deduce that it’s being touched. So you should have far fewer mirror neurons, if any, on this part of your body.

3. The basic idea of the coevolution between genes and culture isn’t new. Yet my claim that a sophisticated mirror-neuron system—conferring an ability to imitate complex actions—was a turning point in the emergence of civilization might be construed as an overstatement. So let’s see how the events may have played out.

     Assume that a large population of early hominins (such as Homo erectus or early H. sapiens) had some degree of genetic variation in innate creative talent. If one rare individual through his or her special intellectual gifts had invented something useful, then without the concomitant emergence of sophisticated imitative ability among peers (which requires adopting the other’s point of view and “reading” that person’s intentions), the invention would have died with the inventor. But as soon as the ability to imitate emerged, such one-of-a-kind innovations (including “accidental” ones) would have spread rapidly through the population, both horizontally through kin and vertically through offspring. Then, if any new “innovative ability” mutation later appeared in another individual, she could instantly capitalize on the preexisting inventions in novel ways, leading to the selection and stabilization of the “innovatability” gene. The process would have spread exponentially, setting up an avalanche of innovations that transforms evolutionary change from Darwinian to Lamarckian, culminating in modern civilized humans. Thus the great leap forward was indeed propelled by genetically selected circuits, but ironically the circuits were specialized for learnability—that is, for liberating us from genes! Indeed, cultural diversity is so vast in modern humans that there is probably a greater difference in mental quality and behavior between a university professor and (say) a Texan cowboy (or president) than between the latter and early H. sapiens. Not only is the human brain phylogenetically unique as a whole, but the “brain” of each different culture is unique (through “nurture”)—much more so than in any other animal.

CHAPTER 5 WHERE IS STEVEN? THE RIDDLE OF AUTISM

1. Another way of testing the mirror-neuron hypothesis would be to see if autistic children do not show unconscious subvocalization when listening to others talking. (Laura Case and I are testing this.)