How we make memories

Some memories are meant to be retained for only a short period and then discarded. By contrast, memories that are more important are kept in the brain's equivalent of long-term storage and can be retrieved at will.

Memory is not perfect, no matter what age you are. Just as you delete old emails on your computer to make way for new ones, the brain routinely prunes out unneeded information, making way for newer, more relevant memories. Moreover, memories are not like snapshots that are taken and then stored in their entirety in one place in the brain. Rather, memories are encoded in a fragmented manner and distributed throughout different parts of the brain. When you remember something, your brain must retrieve and reassemble these different pieces, creating the possibility that you may reconstruct the memory imperfectly.

Brain researchers use a three-stage model to describe how the brain learns and remembers information.

Stage 1: Acquisition

In the acquisition phase, new information is learned or acquired and takes the form of temporary pathways of nerve cell activity in the brain, as one neuron communicates with the next. The location of these neuronal pathways depends on the nature of the information. For example, in most people, activities such as speaking and writing activate neurons in the left temporal lobe, which processes language, whereas studying a map activates neurons in the right parietal lobe, which processes spatial information.

Stage 2: Consolidation

In the consolidation phase, a memory becomes long-term when the initial neuronal pathway becomes strengthened by the hippocampus. Several factors influence whether the hippocampus responds to the newly acquired information and gives the signal to store it as a long-term memory. For example, you are more likely to retain new information if it relates to long-term memories you already have, because the richer associations help you store the information more deeply. This is called declarative memory.

In contrast, procedural memory does not depend on the hippocampus and consolidates memories based on practice. Learning to dance is a good example. Procedural memories are stored throughout the brain in regions that are important for coordinating movement or sequential processing: the frontal lobes, cerebellum, and basal ganglia.

Stage 3: Retrieval

The retrieval phase is the act of recalling a memory. A memory is stored in the brain as a unique pattern of nerve cell activation. When you are not thinking about the memory, its neuronal pattern is inactive. To retrieve the memory, your brain must reactivate the pattern. Similar memories have partially overlapping patterns of neuronal activation. Sometimes when you try to retrieve one bit of information, a similar memory comes to mind and blocks out the information you want.

How memory works

Certain kinds of information can be memorized only if you concentrate, whereas other kinds of memories, such as the faces of people you see regularly and the steps of simple everyday routines, are absorbed without conscious effort.

Researchers and neuroscientists have devised several classification systems to describe the various forms of memory. One major system relies on duration, making a distinction between short-term memories, which are fleeting, and long-term memories, which can persist for a lifetime.

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Keeping track of the steps as you follow a recipe is one example of the use of working memory to complete a task.

Short-term memory

This is information that the brain stores temporarily, encompassing what you need to remember in the next few seconds or minutes. One form of short-term memory is working or immediate memory, which enables you to manipulate information - for example, to complete a task - while holding it in short-term storage. Items in working memory turn over at a high rate because new ones are continually replacing them.

Research shows that the average person can hold only five to nine unrelated bits of information in mind at one time. Without conscious effort to retain it, this information will disappear from your memory within 18 to 20 seconds. Normal aging causes a decline in working memory skills, as do conditions that damage neurons. The relatively transient nature of your short-term memory is actually beneficial, because it allows you to discard information that's no longer needed.

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The ability to ride a bicycle is one example of procedural memory, which stores information related to motor skills at a subconscious level. Procedural memory often persists even in someone with early dementia.

Long-term memory

Most short-term memories quickly decay. However, the brain retains emotionally compelling or personally meaningful events in long-term memory. Although the brain can juggle only a small number of short-term memories at a time, it can store an enormous number of long-term memories. Barring disease or injury, you can always learn and retain something new.

There are several types of long-term memory. One type that is particularly durable even in people who are in the early stages of dementia is procedural memory, which catalogs information related to motor skills. Procedural memory is stored on the subconscious level and allows a person to perform functions such as walking, brushing teeth, and riding a bike.