A game based on Simon shows how people mentally rehearse new information

A brain at rest isn’t always resting. Sometimes it’s rehearsing information it just learned.

For the first time, scientists have watched this mental replay in two human volunteers. These neural ruminations, described May 5 in Cell Reports, might play a role in making a new, fragile memory more durable, scientists suspect.

Most examples of mental replay, in which nerve cells fire off signals in a sequence that matches that of the original learning, come from animals other than humans (SN: 10/3/19). But tests of two paralyzed men participating in the BrainGate2 clinical trial offered a way to observe this rehearsal in humans.

In the study, electrode arrays were implanted in the participants’ brains and linked to computers, with the goal of developing ways to allow people’s thoughts to control computer cursors and other devices. Two volunteers played a copycat game similar to Simon, in which four colored quadrants, each with a different sound tone, light up in a specific sequence. In this case, participants’ brain activity moved a cursor, copying the sequence.

Every so often, researchers sneaked in patterns that repeated more often than the others. Compared with less common sequences, these familiar sequences were more likely to have sparked neural replays in participants’ brains during post-game rests, Jean-Baptiste Eichenlaub of Massachusetts General Hospital and Harvard Medical School in Boston and colleagues found.

This replay happened during sleep and quiet resting. During these repeats, the neural patterns corresponding to the Simon pattern stayed the same, but the sequences could be faster or slower than the original. Scientists don’t yet know whether these mental rehearsals actually strengthen memories.