Why Do Memories of Vivid Dreams Disappear Soon After Waking Up?

Why can most people remember a color, but only a few can remember pitch? —David Hardie, Perth, Australia

Robert O. Duncan, a behavioral scientist at York College, the City University of New York, responds:

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ALTHOUGH MOST of us believe we are better at identifying colors than sounds, our ability to identify the exact frequency of light associated with a color is actually no better than our ability to name a pitch.

Our perception of visible light depends on context. You might go shop-ping for house paints, for example, and be shocked to find that the particular shade of white you selected in the store makes your kitchen look pink! You may have chosen the wrong shade of white because the ambient light in the store differs from that of your home. If we could accurately identify colors, we would never make such mistakes. People may think they are more adept at identifying colors, however, because they tend to associate hues with specific objects, which do not change. For instance, we will generally perceive an apple to be red because the light reflecting off its surface remains fairly constant from moment to moment.

In contrast, in hearing we identify objects, people and speech by the changes in frequency. For example, we can understand a sentence whether it is spoken by a girl with a high voice or a man with a low voice because the relative changes in frequency that occur as the girl and man recite the same words are about the same. In fact, speech and other sounds in

the environment are always changing, which is likely why we have evolved to recognize changes in frequencies rather than any single pitch.

Although few people develop perfect pitch—the ability to precisely name the frequency of a sound—we have a remarkable ability to discriminate among different sounds. We can distinguish house cats from tigers, bicycles from motorcycles, and basketballs from Ping-Pong balls. We use the melodic properties of speech to discriminate a person’s gender, identity and mood. We have an expansive musical memory that enables us to recall tens of thousands of melodies with ease. And with a modest degree of training, most musicians can develop relative pitch, the ability to identify an unknown tone in relation to a known tone.

Why do memories of vivid dreams disappear soon after waking up? —Gil Greengross, via e-mail

Ernest Hartmann, professor of psychiatry at Tufts University School of Medicine and director of the Sleep Disorders Center at Newton-Wellesley Hospital, explains:

WE FORGET almost all dreams soon after waking up. Our forgetfulness is generally attributed to neurochemical conditions in the brain that occur during REM sleep, a phase of sleep characterized by rapid eye movements and dreaming. But that may not be the whole story.

Perhaps the most compelling explanation is the absence of the hormone norepinephrine in the cerebral cortex, a brain region that plays a key role in memory, thought, language and consciousness. A study published in 2002 in the American Journal of Psychiatry supports the theory that the presence of norepinephrine enhances memory in humans, although its role in learning and recall remains controversial.

A lack of norepinephrine, however, does not completely explain why we forget dreams so easily. Recent research suggests that dreaming lies on a continuum with other forms of mental functioning, which are all characterized by activity in the cerebral cortex. On the one side of this continuum is concentrated, focused thought; dreaming and mind wandering lie on the other, with some overlap among the types. The dreaming/reverie end involves some of the most creative and “far out” material. This type of less consciously directed thinking, however, is not easy to remember. Can you recall where your mind wandered while you were brushing your teeth this morning?

In general, we are very good at forgetting nonessentials. In fact, many of our thoughts, not just those we have while dreaming, are lost. We tend to recall only things that we think about often or that have emotional significance—a problem, a date, a meeting. Mulling over important thoughts activates our dorsolateral prefrontal cortex (DLPFC), a brain region that facilitates memory.

Although most dreams vanish, certain ones tend to remain. These dreams were so beautiful or bizarre, they captured our attention and increased activity in our DLPFC. Thus, the more impressive your dream or thought, the more likely you are to remember it.

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