The sea slug Aplysia californica is not unlike an eggplant. It is big—up to a foot long and six pounds—and bruise-purple from gorging on seaweed. Harass one, and it will emit “a very fine purplish-red fluid,” as Charles Darwin found long ago, “which stains the water for the space of a foot around.” Hardly a jewel of the sea.

Yet neuroscientist Eric R. Kandel looked at the slug 50 years ago and saw a gemlike formal simplicity, which he used to help build the foundations of modern neuroscience. With Aplysia, Kandel revealed that we learn not by altering neurons but by strengthening or building new synapses, or connections, between them—a breakthrough of a lifetime. Then he went on to elucidate the most intricate and basic mechanisms underlying this vital process, including how this synaptic remodeling embodies the concept now known as gene expression; that is, it occurs because genes, along with shaping our bodies and coloring our hair, constantly alter our brains by responding to experience.

This article was originally published with the title Eric Kandel: From Mind to Brain and Back Again.

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