How does the brain form and retain memories? This question has fascinated people from all walks of life from time immemorial. Although scientists have discovered a tremendous amount about brain function over the past century, the biological basis of memory largely remains a mystery. Part of the problem, however, is the nature of the question itself. This leads to a more basic question: what does the term memory really mean?
In the scientific realm, the term 'memory' encompasses far more than a person's ability to recall facts or past events. This ability is called declarative memory. The main subdivisions of declarative memory are short term and long term memory. Anyone who has ever pulled an all nighter cramming for a final exam is well aware of this distinction. In contrast, remembering how to perform a series of motor skills is known as procedural memory. Spatial memory is rather tricky; it combines some aspects of declarative memory with a mental map of the external environment. The ability to keep track of long term goals is the hallmark of executive memory. This article will touch on each aspect of memory and hopefully shed some light on this phenomenon.
In some respects, short term memory is the most straightforward aspect of declarative memory to study. The vast majority of information processed by the brain probably makes it no farther than the short term memory. For example, if you see an unfamiliar phone number on TV, you can usually remember it long enough to jot it down. If a few minutes elapse without you seeing the number or mentally repeating it, however, you will most likely forget it. Another good example is trying to remember what you ate for dinner last night as opposed to last week. Unless last week's dinner was a special occasion (or you eat the exact same meal every night), you probably will not remember the details of the event; however, you probably will have a vague recollection of the event itself.
Clearly, some information makes it through the filter of short term memory and ends up in the vast reservoir of long term memories. In 1949, a neuroscientist named Donald Hebb proposed that the key to understanding memory was a process called LTP, or long term potentiation. This concept is often expressed as the maxim "neurons that fire together wire together." By this Hebb meant that repeated stimulation of two neurons reinforces their synaptic connections, resulting in stronger responses to subsequent stimuli. Conversely, a lack of stimulation leads to weakened synaptic connections, and sometimes to their complete disappearance.
At the time, the idea that neuronal synapses exhibit plasticity throughout adult life was a fairly radical concept. Some scientists were skeptical of the notion that brain circuits were malleable after infancy. Others were intrigued by the concept but were unsure where in the brain to look. A few years later, an important clue would appear in the form of Henry Molaison, known by his initials H.M., whose unique situation cemented the role of the brain's hippocampus in consolidating short term information into long term memory.
One of the most famous patients in the annals of medicine, H.M. (1926-2008) was a dramatic example of the consequences of bilateral destruction of the hippocampus. After sustaining a head injury in childhood, H.M. developed temporal lobe epilepsy, which grew worse over time. When no medications helped, he underwent radical brain surgery as a last ditch effort to control the seizures. Employing a technique called electrical ablation, surgeons destroyed most of H.M.'s deep temporal lobes, and with them his hippocampi.
For readers interested in etymology, the hippocampus earned its name due its vague resemblance to a seahorse. Hippocampal neurons are connected to several parts of the brain involved with emotions and arousal including the hypothalamus and amygdala. Axons called Schaffer collaterals form a strong excitatory circuit within the hippocampus itself. Although this part of the brain readily undergoes LTP, the possibility of an out of control excitation loop means that the hippocampus is also the most likely place for a seizure to originate.
The operation successfully ended H.M.'s seizures; unfortunately, it also ended his ability to form long term memories. H.M. developed severe anterograde amnesia immediately after the operation, to the extent that he was unable to remember any new information for longer than a few seconds. When asked his age, H.M. would consistently underestimate it. When asked his place of residence, H.M. could only remember his childhood address. Interestingly, H.M.'s procedural memory remained intact. For instance, he became adept at tracing figures; on the other hand, he never recalled practicing this activity, let alone having been asked to learn this task in the first place.
In contrast to short term memory, surprisingly little is known about long term declarative memory. The main points of agreement are, first, the brain somehow sifts through the vast stream of conscious experience, retaining information that seems interesting, important, or both. Second, information encountered repeatedly can be overlearned to the point that forgetting it becomes virtually impossible.
For example, nearly all Americans can identify the figure on the front of the one dollar bill as George Washington. When asked about Washington's significance, the invariable response is that he was the first President of the United States. (As an aside, almost no one distinctly remembers the first time s/he learned these two facts). Even people with severe dementia tend to remember the name of the first U.S. president. A general consensus has emerged that long term memories (or overlearned facts at least) seem to be stored in multiple areas of the cerebral cortex. In light of these observations, no alternative explanation seems plausible.
Procedural or motor memory seems to be the domain of the cerebellum. Specifically, a process called LTD (long term depression) in the Purkinje cell layer of the cerebellar cortex seems to fine tune motor coordination. In practical terms, this means that when it comes to riding a bicycle or playing video games, individual skills improve to a point then level off. Cerebellar damage often results in loss of fine motor control, dysequilibrium, as well as apraxias, or difficulty completing a planned series of movements.
Spatial memory. The most widely used method to assess an animal's spatial memory is a test called the Morris water maze, which consists of a small container filled with cloudy water and a submerged platform on which a mouse can fit. Normal mice figure out the platform's location after a few minutes and remember its whereabouts during subsequent trials. Drugs that block protein synthesis, however, also block a mouse's ability to remember the platform's location.
In humans the brain's right parietal lobe is especially important in spatial memory. Some stroke victims experience a condition called parietal neglect syndrome. When given a circle and asked to draw a clock face, they crowd the numbers into one half of the circle. Many patients with parietal neglect shave only half of their face. Apparently, in their brains' mental map, half of the world has ceased to exist.
Executive memory, in a sense, is the most difficult form of memory to pin down. One example would be asking a student the type of degree s/he expects to receive upon graduating from a 4 year undergraduate program. Almost all would reflexively answer "a bachelor's degree," even though they might go days at a time without thinking about school, or for that matter attending class. The main insights into executive memory come from rare cases of patients with dissociative or psychogenic fugue. This term describes people who, after intense emotional stress or trauma or sometimes for no apparent reason, abandon their job, friends, and family; in essence their identity evaporates. They are sometimes rediscovered weeks or months later in a distant location with a different identity and no recollection of the events leading to their disappearance.