If you give a bioengineer a cookie…
“When you grab a cookie and want to break off a piece with a chocolate chip,” says Maurice Smith, balancing a crumbly bit between two of his fingers, “your brain must represent that action plan extrinsically, as it is an activity based in the world.”
The cookies are on hand to celebrate the bioengineer’s birthday in his lab at 60 Oxford Street, a white squat building located on the northernmost edge of the Harvard campus. A half moon of chocolate cake with a line of colored candles still intact also sits nearby.
Gesticulating with the cookie, Smith, Associate Professor of Bioengineering at the Harvard School of Engineering and Applied Sciences (SEAS), further teases out the intricacies of motor memory.
“An intrinsic representation is one that’s body-based and procedural. It relates to the complex series of muscle and joint movements your body has to make to complete a task,” Smith says.
“When I first had the thought to grab the cookie and rip off a chunk with a chocolate chip, my body responded appropriately,” he notes.
Understanding the way the brain represents extrinsic and intrinsic actions, and the relationship between the two, has been of great interest to researchers who seek to understand motor control and motor learning—or, put simply, how we learn to move.
Just a few months ago, Smith and his colleagues in the Neuromotor Control Lab laid out a generalizable theory about how the brain encodes such motor memories. Writing in the Journal of Neuroscience, they showed that units of motor memory are not so binary after all, but instead a mixture of both the intrinsic and the extrinsic.
“There’s no question that our actions are inherently spatial, but the nature of the coordinate frame used in motor memory to represent space for action planning has been hotly debated,” explains Smith. “The predominant idea had been that in memory we maintain separate intrinsic and extrinsic representations of action and translate between the two when necessary. But our work shows that memory representations are combinatorial rather than separate.”
Individual neurons in several different motor areas of the brain encode multiplicative combinations of intrinsic and extrinsic representations, a property that neurophysiologists have called gain-field encoding. This much was known before, but it was thought that gain-field encoding simply provided a way to translate between intrinsic and extrinsic representations.
“We found that this gain-field encoding, which leads to a combinatorial representation of space, is not simply an intermediary in the transformation between representations, but is in fact the encoding on which motor memories are based,” says Smith. “This suggests that the neurons which display gain-field encoding are the same ones that store the motor memories associated with the actions we learn.”
