Runners, on your mark // Spectators, feel your pulses rising // The brain’s to blame.
The macaque was wired for observation, with electrodes implanted in its frontal lobe (the premotor cortex) to record the activity of motor neurons. Those are the first nerve cells to fire, or transmit electrochemical signals, in a cascade of neural impulses that control muscle contractions. In this experiment, Giacomo Rizzolatti and his colleagues at the University of Parma in Italy were studying how motor neurons trigger hand movements in monkeys. As an animal performed a specified task, such as popping a peanut into its mouth, a tatatatata sound would indicate a neuron had fired.
But this monkey was between tasks, waiting for its next instructions, when a researcher happened to grab something in its view. To the scientist’s astonishment, he heard the sound of the monkey’s motor neurons firing, a sound he should have heard only when the macaque was moving its hand.
At the time of the experiment, in 1992, neuroscientists thought that when each type of neuron fired, it was to initiate a single function. According to this theory, a sensory neuron might release neurotransmitters to begin the brain’s response to sensory input, such as observing and recognizing the peanut, while a motor neuron would fire to trigger the hand muscles to move. But in Rizzolatti’s experiment, it appeared that the same neuron fired when the monkey grabbed a peanut and when he watched the researcher make a similar movement. One neuron had launched electrochemical impulses for both perception and action.
The existence of such a multipurpose brain cell, which came to be known as a mirror neuron, ultimately led to a hypothesis that would explain why, for example, watching a newscast of a sobbing woman walking through the rubble of her former home may move us to tears. Or how the sight of a spider crawling on someone’s shoulder can cause an involuntary shudder, or why, perched at the edge of our seats at a soccer match, our adrenaline and emotions may surge as if we were the ones on the field.
Researchers now think that watching the pitcher for your favorite baseball team, for instance, excites the same neural circuit that’s involved when you wind up for a toss during the company softball game. According to the mirror neuron hypothesis, it’s only when we imitate or mirror people’s actions or expressions in our mind’s eye that we can understand their intentions and recognize and respond to their feelings.