Even when synapses do fire together, they sometimes do not wire together. But not everything we remember results from reward or punishment, and in fact, most experiences are forgotten. The theory proved sound, and the molecular details of how synapses change during learning have been described in detail. This idea gave rise to an oft-quoted axiom: “Synapses that fire together wire together.” In the dogs’ case, it would mean the brain now knows that the sound of a bell is followed immediately by the presence of food.

When this happens, learning has taken place. Hebb proposed that when two neurons fire together, sending off impulses simultaneously, the connections between them-the synapses-grow stronger. In 1949 psychologist Donald Hebb adapted Pavlov’s “associative learning rule” to explain how brain cells might acquire knowledge. Our concepts of how the two and a half pounds of flabby flesh between our ears accomplish learning date to Ivan Pavlov’s classic experiments, where he found that dogs could learn to salivate at the sound of a bell.