But an even bigger puzzle remains. Is language mediated by a sophisticated and highly specialized mental “language organ” that is unique to humans and that emerged completely out of the blue, as suggested by the famous MIT linguist Noam Chomsky? Or was there a more primitive gestural communication system already in place that provided scaffolding for the emergence of vocal language? A major piece of the solution to this riddle comes from the discovery of mirror neurons.
I HAVE ALREADY alluded to mirror neurons in earlier chapters and will return to them again in Chapter 6, but here in the context of evolution let’s take a closer look. In the frontal lobes of a monkey’s brain, there are certain cells that fire when the monkey performs a very specific action. For instance, one cell fires during the pulling of a lever, a second for grabbing a peanut, a third for putting a peanut in the mouth, and yet a fourth for pushing something. (Bear in mind, these neurons are part of a small circuit performing a highly specific task; a single neuron by itself doesn’t move a hand, but its response allows you to eavesdrop on the circuit.) Nothing new so far. Such motor-command neurons were discovered by the renowned Johns Hopkins University neuroscientist Vernon Mountcastle several decades ago.
While studying these motor-command neurons in the late 1990s, another neuroscientist, Giacomo Rizzolatti, and his colleagues Giuseppe Di Pellegrino, Luciano Fadiga, and Vittorio Gallese, from the University of Parma in Italy, noticed something very peculiar. Some of the neurons fired not only when the monkey performed an action, but also when it watched another monkey performing the same action! When I heard Rizzolatti deliver this news during a lecture one day, I nearly jumped off my seat. These were not mere motor-command neurons; they were adopting the other animal’s point of view (Figure 4.1). These neurons (again, actually the neural circuit to which they belong; from now on I’ll use the word “neuron” for “the circuit”) were for all intents and purposes reading the other monkey’s mind, figuring out what it was up to. This is an indispensable trait for intensely social creatures like primates.
It isn’t clear how exactly the mirror neuron is wired up to allow this predictive power. It is as if higher brain regions are reading the output from it and saying (in effect), “The same neuron is now firing in my brain as would be firing if I were reaching out for a banana; so the other monkey must be intending to reach for that banana now.” It is as if mirror neurons are nature’s own virtual-reality simulations of the intentions of other beings.
In monkeys these mirror neurons enable the prediction of simple goal-directed actions of other monkeys. But in humans, and in humans alone, they have become sophisticated enough to interpret even complex intentions. How this increase in complexity took place will be hotly debated for some time to come. As we will see later, mirror neurons also enable you to imitate the movements of others, thereby setting the stage for the cultural “inheritance” of skills developed and honed by others. They may have also propelled a self-amplifying feedback loop that kicked in at one point to accelerate brain evolution in our species.
FIGURE 4.1 Mirror neurons: Recordings of nerve impulses (shown on the right) from the brain of a rhesus monkey (a) watching another being reach for a peanut, and (b) reaching out for the peanut. Thus each mirror neuron (there are six) fires both when the monkey observes the action and when the monkey executes the action itself.
As Rizzolatti noted, mirror neurons may also enable you to mime the lip and tongue movements of others, which in turn could provide the evolutionary basis for verbal utterances. Once these two abilities are in place—the ability to read someone’s intentions and the ability to mimic their vocalizations—you have set in motion two of the many foundational events that shaped the evolution of language. You need no longer speak of a unique “language organ,” and the problem doesn’t seem quite so mysterious anymore. These arguments do not in any way negate the idea that there are specialized brain areas for language in humans. We are dealing here with the question of how such areas may have evolved, not whether they exist or not. An important piece of the puzzle is Rizzolatti’s observation that one of the chief areas where mirror neurons abound, the ventral premotor area in monkeys, may be the precursor of our celebrated Broca’s area, a brain center associated with the expressive aspects of human language.
Language is not confined to any single brain area, but the left inferior parietal lobe is certainly one of the areas that are crucially involved, especially in the representation of word meaning. Not coincidentally, this area is also rich in mirror neurons in the monkey. But how do we actually know that mirror neurons exist in the human brain? It is one thing to saw open the skull of a monkey and spend days or weeks probing around with a microelectrode, but people do not seem interested in volunteering for such procedures.
One unexpected hint comes from patients with a strange disorder called anosognosia, a condition in which people seem unaware of or deny their disability. Most patients with a right-hemisphere stroke have complete paralysis of the left side of their body and, as you might expect, complain about it. But about one in twenty of them will vehemently deny their paralysis even though they are mentally otherwise lucid and intelligent. For example, President Woodrow Wilson, whose left side was paralyzed by a stroke in 1919, insisted that he was perfectly fine. Despite the clouding of his thought processes and against all advice, he remained in office, making elaborate travel plans and major decisions pertaining to American involvement in the League of Nations.
In 1996 some colleagues and I made our own little investigation of anosognosia and noticed something new and amazing: Some of these patients not only denied their own paralysis, but also denied the paralysis of another patient—and let me assure you, the second patient’s inability to move was as clear as day. Denying one’s own paralysis is odd enough, but why deny another patient’s paralysis? We suggest that this bizarre observation is best understood in terms of damage to Rizzolatti’s mirror neurons. It’s as if anytime you want to make a judgment about someone else’s movements, you have to run a virtual-reality simulation of the corresponding movements in your own brain. And without mirror neurons you cannot do this.
The second piece of evidence for mirror neurons in humans comes from studying certain brain waves in humans. When people perform volitional actions with their hands, the so-called mu wave disappears completely. My colleagues Eric Altschuler, Jaime Pineda, and I found that mu-wave suppression also occurs when a person watches someone else moving his hand, but not if he watches a similar movement by an inanimate object, such as a ball bouncing up and down. We suggested at the Society for Neuroscience meeting in 1998 that this suppression was caused by Rizzolatti’s mirror-neuron system.
Since Rizzolatti’s discovery, other types of mirror neurons have been found. Researchers at the University of Toronto were recording from cells in the anterior cingulate in conscious patients who were undergoing neurosurgery. Neurons in this area have long been known to respond to physical pain. On the assumption that such neurons respond to pain receptors in the skin, they are often called sensory pain neurons. Imagine the head surgeon’s astonishment when he found that the sensory pain neuron he was monitoring responded equally vigorously when the patient watched another patient being poked! It was as though the neuron was empathizing with someone else. Neuroimaging experiments on human volunteers conducted by Tania Singer also supported this conclusion. I like calling these cells “Gandhi neurons” because they blur the boundary between self and others—not just metaphorically, but quite literally, since the neuron can’t tell the difference. Similar neurons for touch have since been discovered in the parietal lobe by a group headed by Christian Keysers using brain-imaging techniques.