What Is Memory?
Deep within the brain lies a small, "S"-shaped structure known as the hippocampus (Greek for "seahorse," which it resembles). This relatively small region of the brain plays a major role in the process of forging memories: It instantly evaluates incoming data from the five senses and determines whether to store or discard the information.
But memories are not stored in the hippocampus or, for that matter, in any other single site in the brain. Instead, they are stored throughout the brain, especially in the cerebral cortex (the outer layer of gray matter that constitutes the "thinking" portion of the brain) and in the cerebellum (the fist-sized structure at the base of the brain, beneath the cortex, that coordinates movement and balance).
Scientists' understanding of memory has expanded in recent years. Sophisticated brain imaging procedures now make it possible to watch specific areas of the brain "turn on" as a person performs various types of mental activity. Studies of people with brain injuries also have helped researchers identify the brain structures involved in different aspects of memory. In addition, a neurological test known as an event-related potential, which is a type of electroencephalograph, can pinpoint and measure the brain's electrical activity as the neurons (nerve cells) respond to mental stimulation.
These and other advances have moved memory research beyond the simple concepts of short-term and long-term memory. Today scientists describe four memory systems that process incoming information for storage and retrieval. The systems are episodic, semantic, procedural, and working. Each system is believed to involve several structures within the brain.
The episodic memory system is involved in remembering personal experiences, such as a phone conversation you had yesterday or the movie you watched last week. Problems with episodic memory can make it difficult to learn new information or to recall recently learned information.
The semantic memory system manages the storage and retrieval of general knowledge and facts, such as the number of days in a year or the colors in a rainbow. People with problems in semantic memory may have difficulty in naming common objects or describing a named object.
The procedural memory system allows us to learn activities and skills that will then be performed automatically with little or no conscious thought. Examples are riding a bicycle or driving a car. Loss of long-mastered skills or significant difficulties in learning new skills are signs of problems with procedural memory.
The working memory system governs our ability to pay attention and concentrate, and it enables us to temporarily keep needed information in mind (such as a phone number or the directions to a restaurant). Disorders in working memory make it difficult to pay attention or to learn a multistep task.
Even when these memory systems are working well, some memories will be stored and recalled more easily than others. A memory with a strong emotional component, such as where you were on September 11, 2001, will likely be retained for the rest of your life.
Similarly, information is more likely to be stored properly when it is recognized as important. The brain discards massive bits of insignificant information throughout the day. New information is also more likely to be retained and recalled if it is related to information already stored. The links between the new and old information serve as retrieval clues. The more numerous the links and the stronger the associations, the more accessible and clear the memory will be. However, if the new information is too similar to or too different from an existing memory, it may be discarded. For example, people who eat similar cereals for breakfast each morning may not be able to remember which type they ate that day.