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

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.

Think of what this means. Anytime you watch someone doing something, the neurons that your brain would use to do the same thing become active—as if you yourself were doing it. If you see a person being poked with a needle, your pain neurons fire away as though you were being poked. It is utterly fascinating, and it raises some interesting questions. What prevents you from blindly imitating every action you see? Or from literally feeling someone else’s pain?

In the case of motor mirror neurons, one answer is that there may be frontal inhibitory circuits that suppress the automatic mimicry when it is inappropriate. In a delicious paradox, this need to inhibit unwanted or impulsive actions may have been a major reason for the evolution of free will. Your left inferior parietal lobe constantly conjures up vivid images of multiple options for action that are available in any given context, and your frontal cortex suppresses all but one of them. Thus it has been suggested that “free won’t” may be a better term than free will. When these frontal inhibitory circuits are damaged, as in frontal lobe syndrome, the patient sometimes mimics gestures uncontrollably, a symptom called echopraxia. I would predict, too, that some of these patients might literally experience pain if you poke someone else, but to my knowledge this has never been looked for. Some degree of leakage from the mirror-neuron system can occur even in normal individuals. Charles Darwin pointed out that, even as adults, we feel ourselves unconsciously flexing our knee when watching an athlete getting ready to throw a javelin, and clench and unclench our jaws when we watch someone using a pair of scissors.¹

Turning now to the sensory mirror neurons for touch and pain, why doesn’t their firing automatically make us feel everything we witness? It occurred to me that perhaps the null signal (“I am not being touched”) from skin and joint receptors in your own hand block the signals from your mirror neurons from reaching conscious awareness. The overlapping presence of the null signals and the mirror-neuron activity is interpreted by higher brain centers to mean, “Empathize, by all means, but don’t literally feel that other guy’s sensations.” Speaking in more general terms, it is the dynamic interplay of signals from frontal inhibitory circuits, mirror neurons (both frontal and parietal), and null signals from receptors that allow you to enjoy reciprocity with others while simultaneously preserving your individuality.