May 25th, 1998

You wanna know something that really irritates me? It’s the commonly pedestrian description of dreams. People describe their dreams with so little recognition of the subtlety of the phenomenon. Here are some of my cranky-points:

1. Dreams are not movies! You do not "see" a sequence of visually connected images. Remember, your eyes are closed; when there’s no input to the retinas, there’s no optical sensation. Now, it is undeniable that visual experiences are often reported by dreamers; my suggestion is that what we experience as a visual sensation in a dream is really rather far downstream in the processing sequence. For example, let’s consider an ridiculously simple conceptualization of the sequence of steps in visual sensation:

a. pattern of light falls on retina
b. retina applies quick spatio-temporal pattern recognition and passes the "cleaned-up" image to the visual cortex.
c. visual cortex applies immediate temporal contextual factors and interprets the image, passing the interpretation of the image along to "higher processing" cortex.
d. "Higher processing" cortex integrates higher-level context (emotional, personal, body state) and interprets the already highly-interpreted meme, which can no longer be called an image.

My suggestion is that dreaming does not involve any of the processes in steps a, b, or c; instead, I would suggest that dreaming is confined to step d, which means that the mental events of dreaming are all highly-interpreted "mental thingamabobs" that only *represent* (at several levels of indirection) the visual experience.

In short, to describe the dream-experience as a movie is pushing the metaphorical into the literal.

2. You do not dream in color! Nor do you dream in black and white. Since a dream is not a visual experience, the question of color is irrelevant. Color information is interpreted early in the image-processing sequence -- probably by the visual cortex in step c. If you agree that step c is not primarily involved in dreaming, then there’s not much point in talking about color. What might be present is some of the emotional intensity associated with the experience of color.

3. A dream is not a story! There is no law of psychology that requires the dreaming mind to obey any rules of logic, dramatic structure, or characterization. When we tell a dream as if it were a story, we distort the reality of the dream-experience.

"OK, smart ass, so what is dreaming?" you naturally ask. I’ll not be so rash as to offer a theory of dreaming here. I will instead offer a reason for dreaming and another metaphor -- similar in its weaknesses to the movie-metaphor, but also shedding some light on the process, just as the movie-metaphor does.

I’ll suggest that dreams are the brain analogue of a computer process called "garbage collection". Now, I don’t want to push the brain-computer analogy very far, as it breaks down very quickly. But, at the most fundamental level, the brain and the computer are both concerned with processing information (radically different kinds of information in radically different formats!) In computers, we always differentiate between data that is quite recent and still being worked with, and data that is no longer immediately necessary, but should be retained for possible long-term use. Typically, we store immediate data in one kind of data structure, that tends to be fast to access, but wasteful of memory. Every now and then, we need to clean up the rapid-access memory and file away important data into long-term memory; this process is called "garbage collection." I’ll suggest that the mammalian brain performs a housekeeping function rather like this. The huge stream of information pouring in through our senses is stuffed willy-nilly into whatever place will hold it. After some period of time (say, 12 hours?) the brain gets so cluttered up with all this unsorted information that it can’t handle new information well. When that happens, it shuts down external input (sleeps) and sorts through the pile, discarding much of it and filing away some of it. This housecleaning process is dreaming.

By the way, there’s nothing original in these observations; I have read such hypotheses in many places. I’m not claiming authorship -- I am seconding others and endorsing their ideas.

If this be so, then the process of dreaming is a mentally very "deep" process -- it must lie close to the fundamental operation of the brain. As such, we would expect it to be shared by other creatures with our basic brain structure, and in fact, all mammals engage in such a process. The fact that we sleep in order to dream and not to repair muscle fatigue or other physical needs is demonstrated by the behavior of some aquatic mammals (dolphins) that never sleep in our sense; instead, they shut down one hemisphere for a while and continue operating on the other hemisphere. After all, it’s real tough to find a nice safe bower for a nap in the middle of the ocean.

So here is my alternative metaphor for the process of dreaming. You, the dreamer, are a very young child observing from the top of the stairs a party put on by your parents for their friends. You are aware of all the events taking place in the party, but you have little means to understand them. You know a little bit of language, so you can understand snippets of conversation. You certainly understand facial expression, so you can recognize fragementary aspects of the interpersonal interaction. But you know nothing of sexual flirtation, political contention, or the effects of alcohol, so most of what you see makes no sense whatever. After several hours of seeing this spectacle, you return to your room ("wake up") and try to explain to your twin sibling what happened. Would you not explain the party in the only terms you had a grip on: the facial expressions? Would you not fill in the logical gaps in your representation with the meager stock of human behaviors you understood? A sexual flirtation becomes "he likes her"; a political argument becomes "he doesn’t like him", possibly because "he’s been bad". Drunkenness is interpreted as dizziness or silliness.

I am suggesting that the reality of the dream-experience goes far beyond our conscious ability to grasp, represent, or understand. When we describe the dream-experience in terms of movies, color, stories, and other familiar experiences, we deny the richness of our mental life.