Habits in the brain: what the basal ganglia do

In a lab at MIT, rats learned a maze shaped like a T. A gate opens, the rat runs up the long arm, and a tone tells it which way to turn for a treat.

Ann Graybiel's team put tiny wires in the rats' brains, to pick up the signals of single cells. They listened deep in the middle, in the basal ganglia: a group of parts that helps start and steer what the body does. You have them too.

Each tick under the run is one signal from a cell: scientists say the cell fires. In the first days, the cells fired all through the run. The rat sniffed, slowed, turned its head, and the basal ganglia were busy at every step.

Weeks later, the pattern had changed. The cells fired at the start of the run and again at the end, and went quiet in between. The whole run had become one block, a chunk: once it starts, it plays to the end.

Your evening on the phone starts the same way. The sofa sets off a practised chunk, hand to phone and thumb to screen, while the rest of your mind is free to think about tomorrow's dinner. No wonder you could not remember choosing.

But nobody sat down and taught your basal ganglia that the sofa means the phone. They learned it, one evening at a time. Who taught them, and could you teach them the guitar instead?