Learning is the modification of behavior by experience. Memory is the ability of the nervous system to retain what is learned and experienced. Even very simple animals can learn and remember, but these two abilities are most highly developed in humans. Consider the amount of information associated with learning a language, and then the much greater amount of information that language enables our brains to store and to process. The capacity of memory and the rate at which memories can be retrieved are remarkable features of the human nervous system.

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Learning that leads to long-term memory and modification of behavior must involve long-lasting synaptic changes. A phenomenon that may explain how long-term synaptic changes might arise is long-term potentiation, or LTP. LTP results from high-frequency electrical stimulation of certain identifiable circuits that makes these circuits more sensitive to subsequent stimulation. In contrast, continuous, repetitive, low-level stimulation of these same circuits reduces their responsiveness, a phenomenon that has been called long-term depression (LTD). LTP and LTD may be fundamental cellular or molecular mechanisms involved in learning and memory.

Above the level of the synapse, memory involves interactions between several brain areas, as we discussed in the opener to this chapter and in Investigating Life: Place Cells Reveal Processes of Memory Consolidation during Sleep. Some of the first insights into memory processes came from surgical treatment of patients with severe seizures. That treatment involves destruction of the excessively active brain area triggering the seizures. To find the right area, the surgery is done under local anesthesia, with the patient remaining conscious. As different regions of the brain are electrically stimulated with electrodes, the patient reports the resulting sensations. Stimulation of some regions of the association cortex elicits recall of vivid memories. Such observations provided the first evidence that specific areas in the brain are associated with specific memories and that memory can be attributed to networks of neurons. Destroying a small area of the brain does not completely erase a memory, however, so it is postulated that memory is a function distributed over many brain regions and can be stimulated via many different routes.

You experience several forms of memory everyday. You have immediate memory for events that are happening now. Immediate memory is almost perfectly photographic but lasts only seconds. Short-term memory contains less information but lasts longer—on the order of 10–15 minutes. When you are introduced to a group of several new people, you probably will have forgotten their names in an hour or so if you have not written them down, used them in a conversation, or made a conscious effort to repeat them. Repetition, use, or reinforcement by something that gets your attention (a title such as “President,” for example) facilitates the transfer of short-term memory to long-term memory, which can last for days, months, years, or a lifetime. There is also intermediate-term memory, which lasts about a day.

Knowledge about neural mechanisms for the transfer of short-term memory to long-term memory—memory consolidation—has come from observations of persons who have lost parts of the limbic system, notably the hippocampus. A famous case is that of the man identified as H.M., whose hippocampus on both sides of the brain was removed to control his severe epilepsy. After the surgery, H.M. was unable to transfer information to long-term memory. If someone was introduced to him, had a conversation with him, and then left the room for several minutes, when that person returned, H.M did not recognize him—it was as if the conversation had never taken place. Up until his death 55 years later, H.M. remembered events that happened before his surgery but could not remember postsurgery events for more than 10–15 minutes.

The studies of H.M., other such patients, and experiments on animals and even London taxi drivers show that the hippocampus plays an essential role in the acquisition of declarative memories—those that involve people, places, things, and events. The study featured in the opener to this chapter revealed the mapping of spatial information by neural networks in the rat, and the replay of that information when the rat is resting before or after running a maze. Subsequent experiments revealed the replay of the same spatial information during sleep, possibly revealing a process that is consolidating the memory and transferring it into long-term storage (see Investigating Life: Place Cells Reveal Processes of Memory Consolidation during Sleep).