The brain is a dazzlingly complex web of somewhere around 100 billion neurons, each of which communicates with others through thousands of connections. The idea of manipulating such a complex system to figure out how it works seems, on the face of it, improbable. Yet a few intrepid explorers have set their sights on this inner, cellular jungle. People have come up with ingenious electrical or molecular methods of listening in on or tweaking neurons just so they can decipher how this complex thinking-seeing-bicycle-riding machine operates. The special November/December issue of Scientific American Mind titled “The Future You” describes new technology designed to reveal the brain’s secrets from the inside.
In Pittsburg, one young woman who is mostly paralyzed carries two tiny grids of electrodes in her motor cortex, a brain center that helps orchestrate movement. When she plugs her implant into a computer, these electrodes relay information from her neurons to a robot, enabling her to control a mechanical arm and hand. This woman suffers from a neurodegenerative disorder that has left her unable to control her muscles and she seems to enjoy learning to maneuver a robot with her thoughts. Still it is hard for me to fathom the bravery necessary to allow scientists to experiment on your brain in this way.
An obvious use for such a brain-computer interface would be to give paralyzed people more control over the environment. In addition, however, such devices are helping scientists understand how we learn motor tasks. When a kid learns to ride a bike, the behavioral change is obvious: instead of falling to one side, she can sit upright and pedal the bike forward. What happened in her brain as she improved her skill, however, had long remained largely hidden. But by monitoring the activity of neurons while a person or animal learns a task, scientists can directly observe the changes in neuronal signaling that accompany such learning. In some studies in rats and mice, scientists have discovered that mastering a task involves narrowing the set of neurons recruited to perform a procedure to a smaller, presumably critical set (see “Human Cyborgs Reveal How We Learn”).
Other researchers are outfitting the brains of animals with tiny solar cells, and using light to switch specific networks on or off to figure out what they do. The technology, called optogenetics, involves endowing neurons with molecules that convert light into electrical signals, which is what neurons use to communicate. The molecules are light-driven pumps or channels taken from algae or bacteria that transport positive or negative ions into or out of cells, and thereby either rev up or silence neuronal signaling. Researchers have used such inserted channels to pinpoint an aggression circuit in the brain: shining light on a certain cells endowed with these foreign molecules, thereby activating them, made mice suddenly start attacking other mice. Others have used the technique to pinpoint the neural basis of memory recall. Using optogenetics to silence overactive neurons, scientists have halted seizures in mice. By exciting certain motor neurons in the brain, such light “therapy” has smoothed out the gait of mice exhibiting symptoms similar to those of Parkinson’s disease (see “Light-Sensitive Neurons Reveal the Brain’s Secrets”).
But although surgeons can implant electrode arrays in people, no doctor or scientist can reliably direct a light-sensitive protein to a specific set of neurons in a person. So therapeutically, the main promise of optogenetics rests in its ability to help scientists decipher the aberrations in the neural code that underlie certain illnesses. With more knowledge, researchers can develop better treatments. As electrode and optogenetics experiments reveal more circuits involved in learning and memory, perhaps we will figure out how to enhance those processes as well.