People store experiences in their brains. Later they can recall those experiences. That process of storing and recalling is called memory. People use their memories during every moment of their lives. To have a conversation, people must remember words and ideas. Even to walk, people must remember the movements they learned as babies.
Scientists do not know exactly how memory works. They know that memories cause chemical changes in the neurons (nerve cells) in the brain. The chemical changes create what are called memory paths. These paths can remain in the brain for seconds or for a person's entire life.
One type of memory, motor-skill memory, tells people how to do physical things that they have done before. People use motor-skill memory to ride a bicycle and to use a fork. The first time a person uses a fork, for example, the brain stores the actions involved. The next time that person wants to use a fork, the brain recalls the needed actions. Scientists think that it takes about five hours for the brain to store a motor-skill memory. If people try to learn another skill during that time, they might not remember the first skill.
People use another type of memory, factual memory, to store facts, images, and other information. Factual memory lets people remember faces, numbers, and the experiences that happen to them.
Another type of memory is emotional memory. Strong feelings often create lasting memories. When people have frightening experiences, for example, they may remember them vividly. They also might remember just the emotion. If a dog bites a child, for instance, the child might feel fear around dogs for years.
Most scientists believe that there are two levels of memory: short-term, or working, memory and long-term, or permanent, memory. The levels differ in how long the stored information is remembered. Motor-skill memory and factual memory can be either short-term or long-term memory. Emotional memory is long-term memory.
Short-term memory is the temporary storage of information that a person needs to perform mental tasks. To understand this sentence, for example, a reader must keep the first half of the sentence in short-term memory while reading the second half. Another example is keeping a phone number in mind after looking it up and while dialing. Short-term memory lasts only about 15 to 30 seconds at the most.
Long-term memory is the storage of information for longer periods. A long-term memory can last days, months, years, or even a lifetime. Scientists are not sure why only some facts and experiences are stored in long-term memory. They think that storing information for a long time requires repetition and practice. For example, if a person repeats a phone number to himself or dials it many times, the number can be transferred from short-term to long-term memory. Practice helps musicians remember, without thinking hard, where to place their fingers while playing their instruments.
People can pull out information stored in memory through either recall or recognition. Recall, or retrieval, is the ability to remember or repeat something learned earlier. A student who recites a poem from memory uses recall. Recognition is the ability simply to identify something that has been seen or experienced before. Recognition is easier than recall because it does not require retrieving or repeating information. An example of good recognition and faulty recall is the recognition of a person's face but the inability to recall the person's name.
Patterns are easier to remember than random information. For example, it is easier to remember a poem that rhymes than one that does not. Connecting a new idea to an old memory is another type of pattern that helps people to remember. For example, to remember a new word, a person might think of another word that sounds like it.
Some children and a few adults have an unusually strong visual memory. They can glance at a picture or the page of a book and then, when it is removed, still see the image somewhere before their eyes. The visual images they have can be almost as clear as photographs. For this reason they are often described as having photographic memories. Scientists, however, use the term eidetic memory. A person with an eidetic memory may recall an object either immediately after its removal from sight or after a period of several minutes, days, or even years. Scientists know little about the causes of eidetic memory.
People can have problems with their memories. For example, they might read a book and afterward not remember what they read. They might forget where they put their keys. People also forget meetings and appointments.
A major reason for forgetting is the passage of time. When a memory is not retrieved for days or months, it gradually decays. Scientists believe that a physical process in the brain may cause memories to fade, but they are not sure what it is. The buildup of new experiences as time passes also causes people to forget. As the new experiences are recorded in the memory, they interfere with a person's ability to retain older memories. The opposite is also true—old memories can interfere with the retention of new ones.
People also tend to forget things that they do not practice or review. People who study Spanish in high school, for example, often forget how to say certain Spanish words after their studies end unless they continue to speak the language regularly.
In some cases outside factors cause people to lose their memories. People under stress may become forgetful. An accident or a head injury can make people suddenly forget everything, even their names. That condition is called amnesia. Amnesia can last a short time or a long time. A more serious condition is Alzheimer's disease, which causes people to slowly lose their memories and can eventually lead to death.
Although forgetting generally seems undesirable, it actually serves some important functions. Sometimes the brain forgets information that it no longer needs. Then it is open to learning new information. Forgetting can also offer a person relief when the memory of a painful experience might cause anxiety.
Some scientists, however, question whether people really forget anything. It might just get harder to pull up the memories. The forgotten information might just be stored somewhere that is difficult to reach. A temporary inability to retrieve something that is in memory happens when a person has a word “on the tip of the tongue.” In addition, when a person learns something and then forgets it, it is easier to learn it the second time. This fact suggests that the information was still in the brain somewhere.
