Experiments show that learning occurs in two phases: an initial working memory followed by long-term memory. For example, if you look up a number in the phone book, you will probably remember the number long enough to dial but forget it promptly thereafter. This is working memory. However, if you call the number frequently, eventually you will remember the number more or less permanently. This is long-term memory.

Some working memory seems to be electrical in nature, involving the repeated activity of a particular neural circuit in the brain. As long as the circuit is active, the memory stays. If the brain is distracted by other thoughts, or if electrical activity is interrupted, such as by electroconvulsive shock or by a concussion, the memory disappears and cannot be retrieved no matter how hard you try. In other cases, working memory involves temporary biochemical changes within neurons of a circuit, with the result that synaptic connections between them are strengthened.

Long-term memory, on the other hand, seems to be structural-the result, perhaps, of persistent changes in the expression of certain genes. It may require the formation of new, permanent synaptic connections between specific neurons or the permanent strengthened synapses last indefinitely, and the long-term memory persists unless certain brain structures are destroyed. Working memory can be converted into long- term memory, apparently by the hippocampus, which processes new memories then transfers them to the cerebral cortex for permanent storage.