Eighteen years ago, in a laboratory at the University of Parma in Italy, a neuroscientist named Giacomo Rizzolatti and his graduate students were recording electrical activity from neurons in the brain of a macaque monkey. It was a typical study in neurophysiology: needle thin electrodes ran into the monkey’s head through a small window cut out of its skull; the tips of the electrodes were placed within individual neurons in a brain region called the premotor cortex. At the time, the premotor cortex was known to be involved in the planning and initiation of movements, and, just as Rizzolatti expected, when the monkey moved its arm to grab an object the electrodes signaled that premotor neurons were firing. And then, neglecting to turn off their equipment, Rizzolatti and his team got lunch.

What followed lunch that day was a serendipitous discovery. One of Rizzolatti’s graduate students decided to have an ice cream cone for dessert, which he ate in full view of the wired-up monkey. To his surprise, the electrodes suddenly began to signal a spike in cellular activity in the premotor cortex, even though the monkey was motionless.
After shoveling more food into their mouths, the scientists determined that some of the neurons they were recording from fired when the monkey made certain movements – like bringing a piece of food to its mouth – and when the monkey watched someone make similar movements. In other words, the neurons were mirroring observed actions: when the monkey watched someone perform an action its brain seemed to be simulating neural activity as if the monkey was performing the action itself. These “mirror neurons”, as Rizzolatti later dubbed them, were hypothesized to constitute a brain system responsible for our ability to understand the actions of others. We know about our world because we’ve interacted with it and only by simulating this interaction in our heads can we comprehend the behaviour of someone else.

In 1992, almost a year after the ice cream cone incident, Rizzolatti published a short report in a minor neuroscience journal describing his discovery of mirror neurons. The paper was largely ignored. Then, almost four years later, he published a more detailed account of the finding in the journal Brain that launched a torrent of research – more than 300 papers in the past ten years – into mirror neurons and their properties. As he explained to The New York Times in an interview, "It took us several years to believe what we were seeing.”

Since Rizzolatti’s 1996 paper, studies in primates and humans support the idea that mirror neurons help us understand observed behaviour. One primate study found that mirror neurons were activated simply by the sound of an action, like the ripping of paper, while another found that the mental representation of actions was enough to cause mirror neuron firing. These are important results because they demonstrate a mirror neuron response to the meaning of an action and not just the observation of one.

In humans, brain imaging studies of people with autism – a deficit characterized by an inability to comprehend observed behavior – have shown that autistics have less activity in premotor regions during the observation of actions than normal subjects. And more recently, in a dramatic example of the importance of mirror neurons in our understanding of others, apraxia patients with cortical damage in mirror neuron areas were shown to have difficulty recognizing whether hand gestures, like sticking out a thumb to hitch a ride, were performed correctly.

A new paper by Vittorio Caggiano and colleagues at the University of Tubingen in Germany – Rizzolatti is a co-author - suggests that mirror neurons might also play a role in helping us choose appropriate responses to behaviors we observe. Using similar methods to the original mirror neuron studies in macaques, the latest paper found that some mirror neurons fired when a monkey watched the experimenter grasp an object within its reach, while other mirror neurons fired when the monkey watched the experimenter grasp an object that was out of its reach. These mirror neurons, it seemed, responded differently to observed behavior depending on how far the behavior occurred from the monkey.

The authors of the paper then repeated the experiment but with an important twist: they placed a neck-high wall in front of the monkey, forcing the animal to make an intermediate movement – a reach over the wall in this case – to grasp any object. In this situation, the mirror neurons that responded before only to the observation of an action within the monkey’s reach completely stopped firing. And the mirror neurons that responded only to the observation of an action outside of the monkey’s reach now fired vigorously, no matter where the experimenter grasped the object. The mirror neurons were not simply encoding the distance of an observed act, they were encoding whether the monkey could perform the act without an intermediate behavior.

One can only speculate, of course, as to the behavioral consequences of mirror neurons that differentiate between actions that can be immediately performed and actions that require intermediate behaviors. But an intriguing idea proposed by the paper’s authors is that such a system might help us choose behaviors of our own. If mirror neurons help us understand the act of a basketball player making a jump shot by simulating neural activity as if we were making the jump shot, perhaps, if we don’t have a basketball within reach, they run simulations of behaviors that might allow us to get the ball – calling for a pass or grabbing a rebound. Such neural simulations might help us decide how best to quickly respond to changing events.
Determining whether such mirror neurons exist in humans will require more research, using indirect methods such as functional brain imaging. Even so, this finding suggests how mirror neurons might link the understanding of the behavior of other people with the production of our own behavior, which is a crucial step towards determining the neural processes that caused Rizzolatti’s graduate student to eat an ice cream cone on that fateful day in Italy, eighteen years ago.

Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His latest book is How We Decide.