A baby zebrafish is just half the size of a pea. A recent look inside its transparent brain, however, offers clues to the far bigger mystery of how we remember—and how we forget.

In an experiment that yielded insights into memory and the brain, a team of researchers at the University of Southern California taught the tiny creature to associate a bright light with a flash of heat, a temperature change the fish responded to by trying to swim away. Using a custom-designed microscope, the team then captured images of the animals’ brains in the moments before and after they learned to associate the light and the heat. It’s the first known look at how a living vertebrate’s brain restructures itself as the animal forms a memory.

In the image published with t…