Written by Richard Crews   

Memory is a powerful aspect of human intellect. It links us to our past, allows us to interpret and manage the world around us, and secures our relationships with the people who are important to us.

Memory actually has several different parts. Perhaps the best known--and the one we work hardest at--is called "long-term memory." This is the one built up by the experiences of a lifetime, the one we expand and strengthen through years of schoolwork, the one which fades last and least as our years advance--it is the one which decorates our old age.

Long-term memory seems boundless. Some remarkable people (like Kim Peek) memorize thousands of books word-for-word; many less remarkable people (like you and me) remember billions of bits of information, and dip into that vast encyclopedia every day.

Long-term memory is built by associating sensory and mental elements together in vast, tangled webs of neurons, that is, brain cells. Elements are associated because they have similar sounds, colors, textures, smells--or similar patterns and meanings (although "meaning" is simply the association with mental networks that were laid down previously).

The glue of mental associations is emotion--amusement, titillation, fear, affection, etc. The stronger the emotional glue, the more easily long-term memory associations are made and the longer they are preserved. That is why memory-assistance systems emphasize building images that are colorful, active, silly--at best, obscene or profane. The more emotionally titillating or charged the associations are, the better something is remembered.

Long-term memories are built over a time frame of minutes to hours. They are then reviewed and renewed over days to weeks. They last months to years--sometimes (with renewal at monthly or yearly intervals) they last a lifetime.

Short-term memory functions on a much more abbreviated time frame--over seconds to minutes. Short-term memory is where we initially store ideas and sensory impressions; then we either use them (as with a telephone number we are about to call or a street address we are trying to find), store them in long-term memory (by identifying associations and links), or forget them. All within a few seconds or, at most, a few minutes.

Sensory memory is the third general kind of memory function; it is the least appreciated and hardest to study of the three. Each of the three "acute" senses (that is, the senses whose input may come and go quickly), sight, hearing, and touch, has a quick memory function which seems to be built into the sensory apparatus itself. Thus, for example, if we see someone whirling around in the dark with a bright light (perhaps a kind of firecracker known as a "sparkler"), there is a trail of sparkling lights that follows the trail of the light source. Sometimes this is called an "after image" and is considered related to the fatigue and recovery of the retinal vision cells themselves. But it is more than that. The image and after images precipitate cascades of mental associations seeking to identify and find meaning for the visual image.

In senior years, visual memory (also called "iconic memory") tends to become slow and abbreviated, and the cognitive usefulness of it blunted. This is the reason for the phenomenon of "missed association" in advanced age. A sign flashing by on the highway reads "horses for sale" and the elderly passenger reports seeing "houses for sale." It is also the reason that reading is easier for the elderly in bright light; the trains of associations that head into the brain are more distinct and more likely to find meaningful associations before the blunted iconic memory gives out.

Similarly, auditory input causes "echoic memory" which lets the listener rehear the stimulus for up to three or four seconds. For example, a sudden explosion came and went too quickly for analysis and identification; echoic memory lets us send inward to our brains repeated cascades of neural associations looking for the meaning--was that a gun shot, a car backfire, a fire cracker? What direction did it come from and how far away was it?

As with visual memory, the dulling of echoic memory also causes missed associations. A convention speaker says "paper trail" and the elderly listener reports hearing "vapor trail." And it is the reason the elderly can follow a conversation better at a loud volume and without background noise, better without a foreign accent or slurred speech; the fewer and duller neural cascades that are sent inward need to be clearer and more precise than the ones we needed in our youth. The common decrease in auditory acuity (particularly in the 4,000-cycles-per-second range that accumulates from a lifetime of acoustic trauma) also contributes to this difficulty, but it is not the whole story.

Memory is a many-faceted intellectual resource. Long-term memory, short-term memory, and the three kinds of sensory memory are very different in the ways they function--particularly in their transience and durability--but they all must function together smoothly for an efficient, effective, and satisfying intellectual life.