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

Other researchers have tested our hypothesis using TMS (transcranial magnetic stimulation). TMS is, in one sense, the opposite of EEG: Rather than passively eavesdropping on the electrical signals emanating from the brain, TMS creates electrical currents in the brain using a powerful magnet held over the scalp. Thus with TMS you can induce neural activity artificially in any brain region that happens to be near the scalp. (Unfortunately, many brain regions are tucked away in the brain’s deep folds, but plenty of other regions, including the motor cortex, are conveniently located directly beneath the skull where TMS can “zap” them easily.) The researchers used TMS to stimulate the motor cortex, then recorded electromuscular activation while the subjects watched other people performing actions. When a normal subject watches another person performing an action—say, squeezing a tennis ball with the right hand—the muscles in the subject’s own right hand will register a tiny uptick in their electrical “chatter.” Even though the subject doesn’t perform a squeezing action herself, the mere act of watching the action leads to a tiny but measurable increase in the action-readiness of the muscles that would contract if she were performing it. The subject’s own motor system automatically simulates the perceived action, but at the same time it automatically suppresses the spinal motor signal to prevent it from being carried out—and yet a tiny trickle of the suppressed motor command still manages to leak through and down to reach the muscles. That’s what happens in normal subjects. But the autistic subjects showed no sign of increased muscle potentials while watching actions being performed. Their mirror neurons were missing in action. These results, taken together with our own, provide conclusive evidence that the hypothesis is correct.

THE MIRROR-NEURON HYPOTHESIS can explain several of the more quirky manifestations of autism. For instance, it has been known for some time that autistic children often have problems interpreting proverbs and metaphors. When asked to “get a grip on yourself,” the autistic child may literally start grabbing his own body. When asked to explain the meaning of “all that glitters is not gold,” we have noticed that some high-functioning autistics provide literal answers: “It means it’s just some yellow metal—doesn’t have to be gold.” Although seen in only a subset of autistic children, this difficulty with metaphor cries out for an explanation.

There is a branch of cognitive science known as embodied cognition, which holds that human thought is deeply shaped by its interconnection with the body and by the inherent nature of human sensory and motor processes. This view stands in contrast to what we might call the classical view, which dominated cognitive science from the mid-through late twentieth century, and held that the brain was essentially the same thing as a general-purpose “universal computer” that just happened to be connected to a body. While it is possible to overstate the view of embodied cognition, it now has a lot of support; whole books have been written on the subject, Let me just give you one specific example of an experiment I did in collaboration with Lindsay Oberman and Piotr Winkielman. We showed that if you bite into a pencil (as if it were a bridle bit) to stretch your mouth into a wide, fake smile, you will have difficulty detecting another person’s smile (but not a frown). This is because biting the pencil activates many of the same muscles as a smile, and this floods your brain’s mirror-neuron system, creating a confusion between action and perception. (Certain mirror neurons fire when you make a facial expression and when you observe the same expression on another person’s face.) The experiment shows that action and perception are much more closely intertwined in the brain than is usually assumed.

So what has this got to do with autism and metaphor? We recently noticed that patients with lesions in the left supramarginal gyrus who have apraxia—an inability to mime skilled voluntary actions, such as stirring a cup of tea or hammering a nail—also have difficulty interpreting action-based metaphors such as “reach for the stars.” Since the supramarginal gyrus also has mirror neurons, our evidence suggests that the mirror-neuron system in humans is involved not only in interpreting skilled actions but in understanding action metaphors and, indeed, in other aspects of embodied cognition. Monkeys also have mirror neurons, but for their mirror neurons to play a role in metaphor monkeys may have to reach a higher level of sophistication—of the kind seen only in humans.