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

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.