If humans forgot everything, the consequences would be devastating to their daily lives. It would be impossible to do one’s job—much less find one’s way to work. Individuals who suffer damage to certain brain regions, particularly the hippocampus, experience this kind of significant memory loss, amnesia, which is marked by an inability to create new long-term memories. In addition, some amnesics lose their ability to recall events that occurred before the brain injury, a condition known as retrograde amnesia. Some amnesics do not experience deficits in short-term memory, and in many cases their memory deficits appear to be limited to the acquisition and recollection of new associations. If an amnesic is introduced to a new acquaintance who leaves the room and returns a few minutes later, the amnesic will not remember having met that person. However, amnesics are able to remember some types of new information, though they may be unaware that they are remembering. This was proved in the early 20th century by the French physician Édouard Claparède, who used a pin to prick an amnesic woman each time he shook her hand. Later the patient would not shake hands with Claparède, even though she could not readily explain why. In her case procedural memory effectively helped her avoid the physical pain that accompanied every act of shaking hands with the physician. Such studies demonstrate that procedural memory can function independently of conscious awareness.

Another form of forgetting is associated with the earliest stages of human development: nearly all people lack the ability to retain memories of experiences they had before they were three years old. Known as infantile amnesia, this universal phenomenon implies that the brain systems required to encode and retrieve specific events are not adequately developed to support long-term memory before age three. Another theory points to developmental changes in the means by which memories are formed and retrieved after early childhood, suggesting that the more-developed brain lacks the ability to access such early memories. Sigmund Freud, in contrast, proposed that infantile amnesia was a form of repression—in other words, a defense mechanism against disagreeable or negative recollections.

Researchers have concluded that the infant brain loses memories far more quickly than does the developed brain and that it lacks the ability to generalize to new events. Children under the age of five or six do not yet realize that learning is most effective when new information is associated meaningfully with other knowledge. Young children are similarly unaware that the intentional rehearsal or repetition of new information will enhance their ability to retain it in memory. As children age and develop language expertise, however, they begin to draw upon their semantic memory to help them remember words, facts, and events. They also grow increasingly aware of the ways in which memory serves them. This awareness of how memory works, known as “metamemory,” increases through much of adulthood.

Aging

Older adults experience memory loss, but only for memories of certain types. Episodic memory (the ability to remember specific events) is typically the first type of memory to decline in old age; it is also the last to fully develop in children. Associative memory (the ability to learn, store, and retrieve associations between actions or things) also declines dramatically. In fact, a chief memory complaint among older adults is a decreasing ability to associate a person’s name with his face. Studies conducted separately by American psychologists Marcia K. Johnson and Larry L. Jacoby demonstrated that, whereas older adults are able to remember the gist of an action or event just as well as younger adults, they are unable to recollect the specific details that were involved. Older adults also have particular difficulty remembering the source of their memories, even in cases in which the information is familiar. Yet other types of memory are spared in old age—the most common among these being recognition. It is therefore common for an older adult to recognize a person’s face while failing to recall that person’s name. Jacoby’s work measured age-dependent distinctions between familiarity (recognition) and source memory (recollection) of a given event. His studies provided stronger confirmation that recognition abilities are similar in younger and older individuals, but as people age, they are less able to recall specific details of the events related to the familiar person or thing.

Age-related memory deficiencies can stem from a number of causes. Research in the 1990s by the American psychologist Timothy A. Salthouse suggested that age-related declines in working memory, and in the speed by which information is processed, can reduce a person’s ability to remember specific details of previous events. Changes in the brains of older adults, especially in the frontal lobes and hippocampal area, also may result in age-related memory deficits. More severe and widespread changes in the brain are related to the massive declines in memory functioning seen in Alzheimer disease, also known as Senile Dementia of Alzheimer Type or SDAT (see memory abnormality).