We all carry in our heads various mental representations of our body—one example is the well-known brain map of our sense of touch, sometimes called a homunculus. New studies show how such mental maps blur with age and readily extend to accommodate bionic limbs.
As we age, our sense of touch becomes less accurate—some elderly people have a tough time reading Braille, for example. Looking for the roots of this sensory decline, German researchers at Ruhr University Bochum stumbled on a surprise: rather than shriveling up, the brain’s sensory body map—which helps us discriminate Braille letters by determining where the raised bumps are in relation to one another—expands with age, exactly as it does during learning.
What could explain this paradox? The homunculus is made up of brain cells that represent our fingers, arms, and so on, loosely tracing a distorted human figurine along the cerebral cortex. In younger people the map stays sharp thanks to cells that dampen neural activity between areas representing different body parts. During aging, however, these cells presumably start to slack off; like an ink drawing that someone spills water on, the contours of the body map start to bleed. Luckily, studies show that a fuzzy old homunculus can be brought back into focus by stimulating the fingertips with a special apparatus, allowing at least some recovery of sensory precision.
To the brain, electronic hardware is no different from flesh and blood, suggests a study at the University of California, Berkeley. In the experiment, monkeys learned to control a computer cursor—a stand-in for a bionic limb—through microelectrodes wiretapping their motor cortex. Although this feat is nothing new, the researchers showed for the first time that a stable memory of the new accessory had formed in the brain.
During normal development, a baby learns to control its limbs by creating a mental map of the movable parts of its body—a motor homunculus of sorts. The new finding parallels that process, says neuroscientist Jose Carmena, who led the study, “but it’s about a prosthetic device, and that’s what is profound about it. We’re talking about an extension of your body’s schema.” In other words, once the brain-machine interface gets up to speed, our gray matter might already be set up to achieve effortless, plug-and-play-like control of electronic add-ons.
Note: This story was originally published with the title "Evolving Mental Maps"