With all the futuristic methods and complicated machines in neuroscience, it’s easy to forget that our understanding of the brain rests in the same logical category as sciences of centuries ago, when seemingly regular people had flashes of insight regarding frog’s legs, apples, and clock towers. That is, until you hear a story like this: How Bartul Mimica found specific groups of neurons that are tuned to the postures of our bodies, a new research advancement published this week in Science.
Mimica, a PhD candidate at the Kavli Institute for Systems Neuroscience at the Norwegian University of Science and Technology, was working in the lab one day with his supervisor, who was showing him how to use a technique called single-unit recording to measure how rat neurons respond to motion. A rat with a silicon probe implanted in its brain was sitting on a towel in a flowerpot munching on a cookie. Suddenly, the screen in front of the two men showed one of the rat's neurons firing. Mimica’s supervisor told him: “Look, this cell responds to low velocity.”
Mimica wasn’t so sure. When the rat was eating, he noticed, it was hunched over the cookie, pushing it into his mouth-you’ve seen how rats eat. When the rat stopped eating and relaxed its back, the neuron stopped firing, but the rat still hadn’t changed speed. “It was a simple observation, but it was quite clear even then that there was more to it than saying the cell fires at low velocity,” Mimica says.
So Mimica designed an experiment to determine what the cells were actually responding to. He set up a six-camera system similar to those used in in motion capture for Hollywood animation, tracking the body position of rats in six degrees of freedom-three translations and three rotations around three perpendicular axes. At the same time, he recorded from two different brain areas, the frontal motor cortex and the posterior parietal cortex.
The results showed that the cells were not tuned to motion after all. They were tuned to posture. In particular, specific cells tended to respond to postures that deviated from standard rat position (on four legs, roaming around). For example: If a rat twists his head to the side, hunches his back, or stands on his hind legs, there’s probably a neuron in his brain somewhere going nuts.
This is not such a crazy idea. While you probably don’t consciously think about your posture, your brain needs a constant record of where you are in space. Just consider what happens when something goes wrong in this part of the brain.
“A long time ago-maybe 100 years ago, people who suffered strokes in this region would have symptoms that are neither sensory nor motor,” Mimica says. Patients diagnosed with what was then called optic ataxia could look at an object, identify it, and describe it, but they couldn’t reach out and grab it. Their movement was normal, they just couldn’t get their hand to go in the right direction. “One hundred years ago, people were perplexed by this phenomenon,” Mimica says. “I think we may have found the answer.”
While this is the first study to find cells that respond to different postures, it’s not the first to find that some cells in the brain respond to extremely specific things. Scientists have known since the 1960s that cells in the visual cortex respond to lines in particular orientations-which is how that area of the brain eventually processes edges and shapes.
There is also a set of cells in the temporal lobe that respond only to individual celebrities or people you know very well. They’re called Jennifer Aniston cells and, perhaps surprisingly, were not discovered after a chance run-in with Jennifer Aniston.
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