The Cognitive Hierarchy
When you are awake, you take in physical stimuli through your senses. These stimuli are processed up your cognitive hierarchy. Along the way, the stimuli are abstracted. For instance, you might see the lines making up this 'e' on this page and abstract them as being the letter e. But that is at the lowest levels of the hierarchy. A higher-level example might be when you see a person and then realize that person is your friend.

Dreaming As Imagining
Simply put, dreaming is the reverse of this process. While you are dreaming, stimuli are passed down through your cognitive hierarchy. As the stimuli go down, your mind selects the best specific example of that stimuli. You might start with "enemy" and specify a person who is your biggest enemy.

The same thing is happening when you are awake and are imagining something. Take a moment and imagine you are at the post office mailing a letter. In order to imagine that, you selected things, images, and events that were likely to happen. You created a going-to-the-post-office stimulus at a midway point in the cognitive hierarchy. That stimulus then got processed down until your entire imaginary experience was specified.

Imagining Versus Dreaming
But imagining is unlike dreaming in three ways:

  • In dreams, unexpected things happen.
  • A dream feels real while it last.
  • You generally can't remember your dreams.
To explain why unexpected things happen, it's important to first realize that you aren't consciously aware of your entire cognitive hierarchy. Usually, you are only aware of the middle levels. The low levels process basic stimuli, like the 'e' example above. The middle levels process normal stimuli, like the friend example. And finally, the high levels process abstract stimuli. An example of high-level processing might be taking a series of unfortunate events abstracting them as an embarrassing experience.

To continue then, when you imagine something, you consciously create the plot of what happens at the middle level. That stimulus then propogates down until it is specified in the low levels. On the other hand, when you are dreaming, the plot originates from the high levels. It then propogates through the middle levels and on down to the low levels. But you only consciously experience the stimuli that go through the middle levels. That is why unexpected things happen when you dream.

It's much easier to explain why dreams feel real. When you are awake, and imagining, your imagined plot has to compete with the plot of what is actually going on around you. But when you're asleep, the outside world is cut off (by complex physical mechanisms). This lets you can concentrate on the dreamed plot.

The reason you can't usually remember your dreams is that your long-term memory systems are shut down while you're dreaming. That's why, if you're woken while you're dreaming, you can only remember the last 15 minutes or so of your dream.

John LeFlohic
February 22, 1999