What Are Memories Made Of?

Memory—“perhaps the most extraordinary phenomenon in the natural world,” according to Professor Richard F. Thompson—involves several different functions of the brain. Most students of the brain divide memory into two kinds, declarative and procedural. The procedural involves skills and habits. The declarative, on the other hand, involves storing facts. The Brain—A Neuroscience Primer itemizes memory processes according to the time they take: very short-term memory, which lasts about 100 milliseconds; short-term memory, which is of a few seconds’ duration; working memory, which stores recent experiences; and long-term memory, which houses verbal material that has been rehearsed and motor skills that have been practiced.

One possible explanation of long-term memory is that it starts with activity in the front part of the brain. The information chosen for long-term memory passes as an electrical impulse to a part of the brain known as the hippocampus. Here a process called long-term potentiation enhances the neurons’ ability to pass messages.—See the box “Bridging the Gap.”

A different theory of memory stems from the idea that brain waves play a key part. Its proponents believe that regular oscillations of the brain’s electrical activity, rather like the beat of a drum, help bind memories together and control the moment at which different brain cells are activated.

Researchers believe that the brain stores different aspects of memories in different places, each concept being linked to the area of the brain that specializes in perceiving it. Some parts of the brain certainly contribute to memory. The amygdala, a small almond-size clump of nerve cells close to the brain stem, processes memories of fear. The basal ganglia region is focused on habits and physical skills, and the cerebellum, at the base of the brain, concentrates on conditioned learning and reflexes. Here, it is believed, we store the skills of balance—for example, those we need to ride a bicycle.

A phenomenon called long-term potentiation occurs when neurons are regularly stimulated and release neurotransmitters across the synapse. Some researchers believe that this draws the neurons closer together. Others claim that there is evidence that a message feeds back from the receiving neuron to the transmitting neuron. This, in turn, causes chemical changes that produce yet more proteins to serve as neurotransmitters. These then strengthen the bond between the neurons.

The changing connections in the brain, its plasticity, give rise to the motto, “Use it or lose it.” Thus, to retain a memory, it is helpful to recall it often.