The Tell-Tale Brain

together,” as the old mnemonic goes, eventually even the mere sight of a moving hand (its own or another monkey’s) triggers a response from the command neurons. But if this is the correct explanation, why do only a subset of the command neurons fire? Why aren’t all the command neurons for this action mirror neurons? Furthermore, the visual appearance of another person reaching toward a peanut is very different from your view of your own hand. So how does the mirror neuron apply the appropriate correction for vantage point? No simple straightforward associationist model can account for this. And finally, so what if learning plays a role in constructing mirror neurons? Even if it does, that doesn’t make them any less interesting or important for understanding brain function. The question of what mirror neurons are doing and how they work is quite independent of the question of whether they are wired up by genes or by the environment.

Highly relevant to this discussion is an important discovery made by Andrew Meltzoff, a cognitive psychologist at the University of Washington’s Institute for Learning and Brain Sciences in Seattle. He found that a newborn infant will often protrude its tongue when watching its mother do it. And when I say newborn I mean it—just a few hours old. The neural circuitry involved must be hardwired and not based on associative learning. The child’s smile echoing the mother’s smile appears a little later, but again it can’t be based on learning since the baby can’t see its own face. It has to be innate.

It has not been proven whether mirror neurons are responsible for these earliest imitative behaviors, but it’s a fair bet. The ability would depend on mapping the visual appearance of the mother’s protruding tongue or smile onto the child’s own motor maps, controlling a finely adjusted sequence of facial muscle twitches. As I noted in my BBC Radio Reith Lectures in 2003, entitled “The Emerging Mind,” this sort of translation between maps is precisely what mirror neurons are thought to do, and if this ability is innate, it is truly astonishing. I’ll call it the “sexy” version of the mirror-neuron function.

Some people argue that the complex computational ability for true imitation—based on mirror neurons— emerges only later in development, whereas the tongue protrusion and first smile are merely hardwired reflexes in response to simple “triggers” from mom, the same way a cat’s claws come out when it sees a dog. The only way to distinguish the sexy from the mundane explanation would be to see whether a baby can imitate a nonstereotyped movement it is unlikely to ever encounter in nature, such as an asymmetrical smile, a wink, or a curious distortion of the mouth. This couldn’t be done by a simple hardwired reflex. The experiment would settle the issue once and for all.

INDEPENDENT OF THE question of whether mirror neurons are innate or acquired, let us now take a closer look at what they actually do. Many functions were proposed when they were first reported, and I’d like to build on these earlier speculations.² Let’s make a list of things they might be doing. Bear in mind they may have originally evolved for purposes other than the ones listed here. These secondary functions may simply be a bonus, but that doesn’t make them any less useful.

First, and most obvious, they allow you to figure out someone else’s intentions. When you see your friend Josh’s hand moves toward the ball, your own ball-reaching neurons start firing. By running this virtual simulation of being Josh, you get the immediate impression that he is intending to reach for the ball. This ability to entertain a theory of mind may exist in the great apes in rudimentary form, but we humans are exceptionally good at it.

Second, in addition to allowing us to see the world from another person’s visual vantage point, mirror neurons may have evolved further, enabling us to adopt the other person’s conceptual vantage point. It may not be entirely coincidental that we use metaphors like “I see what you mean” or “Try to see it from my point of view.” How this magic step from literal to conceptual viewpoint occurred in evolution—if indeed it occurred—is of fundamental importance. But it is not an easy proposition to test experimentally.

As a corollary to adopting the other’s point of view, you can also see yourself as others see you—an essential ingredient of self-awareness. This is seen in common language: When we speak of someone being “self-conscious,” what we really mean is that she is conscious of someone else being conscious of her. Much the same can be said for a word like “self-pity.” I will return to this idea in the concluding chapter on consciousness and mental illness. There I will argue that other-awareness and self-awareness coevolved in tandem, leading to the I-you reciprocity that characterizes humans.

A less obvious function of mirror neurons is abstraction—again, something humans are especially good at. This is well illuminated by the bouba-kiki experiment discussed discussed in Chapter 3 in the context of synesthesia. To reiterate, over 95 percent of people identify the jagged form as the “kiki” and the curvy one as “bouba.” The explanation I gave is that the sharp inflections of the jagged shape mimic the inflection of the sound ki-ki, not to mention the sudden deflection of the tongue from the palate. The gentle curves of bulbous shape, on the other hand, mimic the boooooo-baaaaaa contour of the sound and the tongue’s undulation on the palate. Similarly, the sound shhhhhhhh (as in “shall”) is linked to a blurred, smudged line, whereas rrrrrrrrrrrrrrrrr is linked to a sawtooth-shaped line, and an sssssssssss (as in “sip”) to a fine silk thread—which shows that it’s not the mere similarity of the jagged shape to the letter K that produces the effect, but genuine cross-sensory abstraction. The link between the bouba-kiki effect and mirror neurons may not be immediately evident, but there is a fundamental similarity. The main computation done by mirror neurons is to transform a map in one dimension, such as the visual appearance of someone else’s movement, into another dimension, such as the motor maps in the observer’s brain, which contain programs for muscle movements (including tongue and lip movements).

This is exactly what’s going on in the bouba-kiki effect: Your brain is performing an impressive feat of abstraction in linking your visual and auditory maps. The two inputs are entirely dissimilar in every way except one—the abstract properties of jaggedness or curviness—and your brain homes in on this common denominator very swiftly when you are asked to pair them up. I call this process “cross-modal abstraction.” This ability to compute similarities despite surface differences may have paved the way for more complex types of abstraction that our species takes great delight in. Mirror neurons may be the evolutionary conduit that allowed this to happen.

Why did a seemingly esoteric ability like cross-modal abstraction evolve in the first place? As I suggested in a previous chapter, it may have emerged in ancestral arboreal primates to allow them to negotiate and grasp tree branches. The vertical visual inputs of tree limbs and branches reaching the eye had to be matched with totally dissimilar inputs from joints and muscles and the body’s felt sense of where it is in space—an ability that would have favored the development of both canonical neurons and mirror neurons. The readjustments that were required in order to establish a congruence between sensory and motor maps may have initially been based on feedback, both at the genetic level of the species and at the experiential level of the individual. But once the rules of congruence were in place, the cross-modal abstraction could occur for novel inputs. For instance, picking up a shape that is visually perceived to be tiny would result in a spontaneous movement of almost-opposed thumb and forefingers, and if this were mimicked by the lips to produce a correspondingly diminutive orifice (through which you blow air), you would produce sounds (words) that sound small (such as “teeny weeny,” “diminutive,” or in French “un peu,” and so on). These small “sounds” would in turn feed back via the ears to be linked to tiny shapes. (This, as we shall see in Chapter 6, may have been how the first words evolved in our ancestral hominins.) The resulting three-way resonance between vision, touch, and hearing may have progressively amplified itself as in an echo chamber, culminating in the full-fledged sophistication of cross-sensory and other more complex types of abstraction.

If this formulation is correct, some aspects of mirror-neuron function may indeed be acquired through learning,