If you were watching World Series Game 6 when David Freese hit his game-saving two-run triple on a 3-2 pitch in the bottom of the ninth, you may have jumped out of your seat, sloshed beer down your chest, and spewed half-chewed nachos toward the screen. But unbeknownst to you, as the beer fizzed, your brain leapt up, stretched your left arm nearly out of its socket trying to close the air between ball and glove before you slammed your backside into the Gulf logo on Busch Stadium’s right-field wall.

Your brain plucked the ball from the grass, rocketed it toward third and, effortlessly switching allegiances, your brain slid headfirst into the bag. It also trotted home to score the tying run, and in the next few seconds, it waved a white rally towel, spat, and looked glumly over the Ranger dugout fence. Whatever your conscious fan loyalties, your brain couldn’t help playing both sides, all roles. What your eyes see, your brain plays — as best it can, which is, of course, as variable as our actual playing and living.

The evidence that the spectating brain is also a playing brain has been mounting ever since the early 1990s, when a group of neurophysiologists at the University of Parma, Italy implanted electrodes in the brain of a macaque monkey to find out exactly which neurons fired when the monkey grasped a peanut and brought it to his mouth. The electrodes were placed in the monkey’s premotor cortex, the region known to initiate signals that direct muscle movement in both macaques and humans. The researchers hoped that pinpointing the individual motor neurons that fired when the monkey grasped the peanut might lead to therapies that could help brain-damaged humans recover hand function.

The Parma team succeeded in their original brain-mapping goal, but it was an accidental discovery that made Parma a world-renowned source of cutting-edge neuroscience (not to mention hard cheese and cured ham). As science lore has it, a researcher in the Parma lab was eating peanuts one day when he heard a monitor buzz, indicating that the monkey’s peanut-grasping neurons were firing. But the monkey had no peanut. After a moment of puzzlement came the researcher’s “eureka!” moment: Some of the same motor neurons that fire when a monkey performs an action were also firing when he watched someone else perform that action.

In fact, it took several such chance observations and then years of testing before the Parma researchers could believe what they were seeing, because it so violated their understanding of how the brain worked. Neurons in a macaque or human brain that are specialized to plan, select, and initiate our muscle movements should not fire to somebody else’s muscle movements. But since they appeared to be firing, the researchers wondered what the difference was, from our brain’s point of view, between doing and seeing, between playing the game and watching the game, and, most fundamentally, between one’s self and someone else?

The answer, according to subsequent research on both monkeys and humans, is that in a healthy premotor cortex, the difference is about 80 percent. In other words, about one-fifth of the neurons that fire in the premotor cortex when we perform an action (say, kicking a ball) also fire at the sight of somebody else performing that action. A smaller percentage fire even when we only hear a sound associated with an action (say, the crack of a bat). This subset of motor neurons that respond to others’ actions as if they were our own are called “mirror neurons,” and they seem to encode a complete archive of all the muscle movements we learn to execute over the course of our lives, from the first smile and finger wag to a flawless triple toe loop.

When we see a familiar action, our mirror neurons activate, and their firing lasts exactly as long as the observed action. This allows us to instantaneously understand the action, its goal, and even the emotions associated with it, without having to do any inferential thinking about it. If we are watching strenuous action, mirror neurons even provoke a small but measurable uptick in our heart and respiration rate.

In the nearly 20 years since the macaque in Parma gave new meaning to the phrase “monkey see, monkey do,” hundreds of research projects around the world have studied mirror neurons to learn about everything from how we acquire language to how we develop empathy. And if you are still reading this, you have probably started to wonder what mirror neuron research has to say about the experience that brings you to this site: sports spectating.

Common sense suggests that the sports-watching brain might be an ideal laboratory for testing the properties of mirror neurons. Several scientists have explored this idea, but their research has mostly focused on the spectating brains of professional athletes, because they wanted to see if these cells encode even the most highly specialized motor skills. And who has more refined motor skills than world-class athletes?

The most relevant study, published in 2008, was conducted at The University of Rome, where neuroscientists recruited 10 professional basketball players, 10 expert watchers (basketball journalists and coaches), and 10 students who had never played basketball, and made them all test subjects.

In the first part of the experiment, the researchers had all three groups watch film clips of players attempting free throws. The clips were stopped at ten mid-action intervals and everybody was asked to predict the likely success of the free throws. The players made more accurate predictions at every time interval, but their greatest advantage over both expert watchers and inexperienced students was at the earliest intervals, before the balls had even left the players’ hands, when there were no trajectories to watch.

This indicated that athletes were better than the others at understanding cues from the filmed players’ bodies. But the researchers also wanted to know how much their motor systems, primed by their mirror neurons, contributed to reading those cues. So in a second experiment, using a technique called transcranial magnetic stimulation (TMS), which yields the exact timing of neuronal firing, the researchers monitored the patterns of motor system activity in all three groups as they watched free throw video clips.

Everyone’s motor system perked up watching the action, but the students showed a generalized perk-up, while both players and expert watchers showed activity of the specific motor areas involved in shot-taking. What separated the players from the expert watchers, though, was greater excitation of the hand muscles controlling the ball, especially the muscle controlling the angle of the pinkie finger at the instant the ball left the shooter’s hand. There was not necessarily visible movement of the pinkie, but a measurable increase in what’s called “motor evoked potentials,” which signal preparation for intended action. The most unpredictable result: This activation was greatest when players watched the launch of a ball that was going to miss the basket.

Maybe the brains of the spectating players were not just simulating the shot-taking. Maybe they were trying to score. And so, perhaps, are we. We’re just not as good at it.

Although most of the mirror neuron research I found about sports spectating compared samples at fan extremes, such as professional athletes and those with no experience in a sport, the free throw study suggests that there is a sliding scale of mirror neuron response among spectators based on their real-life sports experience.

Marco Iacoboni, a neuroscientist at UCLA and pioneering investigator of the mirror neuron system in humans, says the brain monitoring technology used in the Rome free throw study is best at measuring mirror neurons that are “strictly congruent,” which means they fire at the sight of an action that exactly matches one in the spectator’s motor repertoire. Other mirror neurons are “broadly congruent,” meaning they fire at observed actions that are similar to ones you have performed.

“About two-thirds of our mirror neurons are broadly congruent,” says Iacoboni, “which suggests to me that the purpose of the mirror neuron system is to get you more attuned to the goal of the action than the action itself.”

In sports fan terms, this means if you are watching someone take a free throw, and you have never played basketball, your strictly congruent mirror neurons will not fire, but the broadly congruent ones that remember tossing a crumpled piece of paper into a wastebasket will.

For technical reasons, and to eliminate extraneous factors, most mirror neuron experiments are designed to monitor brain activity while test subjects watch a fairly simple action sequence. Teasing out what these studies imply about the experience of watching an actual game moves us into the realm of informed speculation. Scientists are understandably reluctant to venture far into that realm, but I asked Iacoboni to answer a few questions and respond to speculations of my own about sports spectating.

Having read that Iacoboni is an avid tennis player and fan, I started there. “If you’re watching a tennis match,” I asked, “with a rooting interest in one of the players, do your mirror neurons fire equally to both players?”

“It’s well known that we have imitation biases,” Iacoboni said, “that we tend to imitate people like ourselves. When I’m watching Federer play Djokovic, most likely I’m going to mirror Federer more, because I like him more.”

“But if mirror neurons fire automatically,” I continued, “and they are goal-oriented, wouldn’t that create a natural tendency to keep one’s eye on the ball, and so make you mirror both players?”

“The motor system is strongly oriented toward the goal of an action,” Iacoboni answered. “When someone hits a tennis ball, the goal is to send the ball into a specific sector of space, and because of that, yes, I think there is a tendency to follow the ball. In fact, sometimes I want to focus on a player, watch how he moves, especially the balance of the body, the posture, but I can’t help myself from following the ball, and I get upset with myself.”

Switching to Iacoboni’s other favorite sport, soccer, I asked if a spectator without much playing experience was likely to have more mirror neuron activity when the ball was near the net, because everyone understands the goal of the action there — get the ball in or keep the ball out — while the goal of midfield action is less obvious to a non-player.

“This is one I don’t know how to answer,” Iacoboni said. “I would like to do the experiment, look at the difference in brain activity near the goal and at midfield. My hunch is that you are right.”

That strong goal orientation, I speculated, might make our mirror neurons show a preference for certain player positions that see more on-the-ball action, like quarterbacks, wide receivers, and running backs in football, who already exert a strong pull on our mirror neurons, because even if we have never played football, most of us have at least “broadly congruent” experiences of running, tossing, catching, and evading the attack of large men who want to harm us.

Iacoboni thought that was a reasonable hypothesis, especially with regard to quarterbacks, but didn’t expand, confessing little interest in American football, because, compared to soccer, it was “so slow, with all that stopping.”

Moving deeper into speculative territory, I asked if our mirror neurons, when activated strongly enough to set off MEPs (the aforementioned motor evoked potentials), could make us feel younger because those potentials, unlike our actual motor skills, could still be at or close to their best.

“In real life, when you make a full-blown action, you get all this sensory feedback,” Iacoboni said, “but when it’s just these tiny muscle contractions or maybe just increased excitability of your motor cortex, your motor plans can go unchecked by sensory feedback in a way that, it’s possible, makes you feel younger. That’s a nice idea.”

Doing an informal study of my own unrealistically youthful mirror neurons before talking to Iacoboni, I watched some Monday Night Football and noticed that I felt more intensely involved in the game when I turned off the sound, so I asked if announcers got in the way of our mirror neurons by engaging us in constant analysis.

“Absolutely,” Iacoboni said. “We actually have some data on that. Being analytical almost shuts down your motor cortex, doesn’t let you simulate in full what you are watching. I shut the sound off while watching tennis.”

Analytic language may put the damper on our brain’s motor areas, but that is not the case with other kinds of language. Research using football and hockey players produced evidence that mirror neurons are activated not only when we see or hear action, but also when we listen to or read descriptions of it. And as with the response to sights and sounds, the degree of mirror neuron activation, which speeds up comprehension, depends on our individual experience with what’s being described.

That research was conducted at the University of Chicago’s Human Performance Lab, where cognitive psychologist Sian L. Beilock is the principal investigator. Beilock explains it like this:

“Even when reading, one of the ways we understand information is by calling on those motor systems we used to act in the past. So for someone with a lot of experience playing, reading about football, for instance, has what we call ‘motor resonance,’ because they can call upon the parts of the brain they used playing to simulate the action in their heads, and that often makes what they’re reading easier to follow and makes them like that information better.”

And this enhanced comprehension is not just a matter of their having heard or read a lot of football language, because they were players?

“Background knowledge certainly aids comprehension,” Beilock answered, “but our brain also reuses motor experience we learned doing things to comprehend what we are watching or reading about. All of our experiences of doing in the world contribute to our comprehension of the world. It turns out, for instance, that we reuse areas of the brain that control our fingers when we count in our head, because at one point we learned to count on our fingers, and those same areas of the brain come alive again. ”

You mean every motor skill we master makes us smarter?

“Yes, in the sense that the better you can do it, the better you can perceive it,” Beilock said, adding that her research team has a National Science Foundation grant to see if motor experience can be used to help students learn physics. “The idea is that if you get a student to physically experience being part, for instance, of an angular momentum system, to experience forces and torques and have to counteract them to keep something stable, it might help them understand these concepts when they’re reading about them. If the experience is grounded in your body, you would call on those motor areas of your brain when you’re listening to a lecture and it could facilitate understanding.”

I guess this means that the evolutionary purpose of our mirror neuron system is not primarily to make us more engaged sports spectators, although ESPN has certainly profited from this major side effect. It’s more likely designed to advance the species by helping us develop cognitive skills, including but not limited to predicting the success of free throws.

After reading the research and talking to Iacoboni and Beilock, I still had an unanswered question. What had launched me on this particular quest for information was the subjective experience of feeling almost physically active when I watched sports or dance, as if I were running, leaping, dodging, punching, and crashing to the ground myself. It was as if from my couch potato position I had reaped the benefit of at least a portion of the endorphins a workout would produce. But nothing in what I read about mirror neurons indicated whether or not we liked it when our mirror neurons fired.

So in a last phone call, I asked Iacoboni if mirror neuron activity made us feel good.

“Oh, yeah,” he said. “Although this is something very hard to prove, it makes a lot of sense. If this system is important for the things we believe it is, which is social cognition, understanding the mental states of others, empathizing with others, and how to learn things just by watching others, evolution must have devised something that makes us feel good when we activate these cells, which makes us do it more and more, because that is an adaptive mechanism. The popularity of watching sports is a good demonstration of this.”

Le Anne Schreiber is the former editor-in-chief of Womensports Magazine and a former sports editor at the New York Times. She was ESPN’s ombudsman from 2007 to 2009 and is the author of two books, Midstream and Light Years. This is her first piece for Grantland.

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