Dreams make perfect sense when you’re having them. Yet, they leave you befuddled the next morning, wondering “where did that come from?” The answer may lie in the dreams of people with amnesia, researchers report in Friday’s issue of Science.

Much of the fodder for our dreams comes from recent experiences. For this reason, scientists have tentatively supposed that the dreaming brain draws from its “declarative memory” system, which includes newly learned information.

The declarative memory stores information that you can “declare” you know, such as the square root of nine, or the name of your dog. Often, you can even remember when or where you learned something - for example, the day you discovered the harsh truth about Santa Claus. That’s called episodic memory.

People who permanently suffer from amnesia can’t add new declarative or episodic memories. The parts of their brains involved in storing this type of information, primarily a region called the hippocampus, have been damaged. Although amnesiacs can retain new information temporarily, they generally forget it a few minutes later.

If our dreams come from declarative memories, people with amnesia shouldn’t dream at all, or at least dream differently than others do. But new research directed by Robert Stickgold of Harvard Medical School suggests quite the opposite.

Just like people with normal memory, amnesiacs replay recent experiences when they fall asleep, Stickgold’s study shows. The only difference seems to be that the amnesiacs don’t recognize what they’re dreaming about.

Dreaming of Tetris

Every day, the people in the study played several hours of the computer game Tetris, which requires directing falling blocks into the correct positions as they reach the bottom of the screen. At night, the amnesiacs didn’t remember playing the game. But, they did describe seeing falling, rotating blocks while they were falling asleep.

A second group of players with normal memories reported seeing the same images.

Therefore, Stickgold’s research team concluded, dreams must come from the types of memory amnesiacs do have, which are called “implicit memories.” These are memories that scientists can measure even when individuals don’t know that they have them.

One class of implicit memories is found in the procedural memory system, which stores information that you use without really being able to say how you know what you’re doing. When you ride a bicycle for the first time in years, or type on a keyboard without looking, you’re relying on procedural memory.

Another type of implicit memory uses “semantic” knowledge, and resides in different parts of the brain, including a region called the neocortex. Semantic knowledge involves general, abstract concepts. Both groups of Tetris players, for example, only described seeing blocks, falling and rotating, and evidently did not see a desk, room, or computer screen, or feel their fingers on the keyboard.

Without help from the hippocampus, new semantic memories are too weak to be intentionally recalled. But they can still affect your behavior - for example, causing you to buy a certain brand of something you saw in an advertisement you don’t remember.

In contrast, the information in episodic memories is associated with specific times, places or events. Without these “anchors” to reality, it’s no wonder that dreams are so illogical and full of discontinuity, the study’s authors say.

Stickgold believes that dreams serve a purpose for the brain, allowing it to make necessary emotional connections among new pieces of information.

“Dreams let you consolidate and integrate your experiences, without conflict with other input from real life,” Stickgold said. “Dreaming is like saying, ‘I’m going home, disconnecting the phone, nobody talk to me. I have to do work.’”

Because the hippocampus seems to be inaccessible for this “off-line” memory processing, the brain may use the abstract information in the neocortex instead.

According to Stickgold’s theory, dreaming is like choosing an outfit by reaching into bins labeled “shirts,” “pants” and so on. You’ll rummage up something to wear, but it won’t be a perfectly matching ensemble.

Sleep's earliest messages

The period of sleep that Stickgold’s team studied is called “hypnagogia.” It’s an in-between state between being fully awake and fully asleep. Many people who have just had an intense new experience of some kind, either mental or physical, often report replays of that experience during this stage.

In his poem, “After Apple Picking,” for example, Robert Frost describes seeing the apples and apple blossoms, and feeling the ladder sway as he nods off to sleep. Stickgold’s first encounter with this phenomenon occurred after a day of mountain climbing, when he felt the sensation of rocks under his fingertips as he fell asleep.

Hypnagogic sleep is different from REM sleep, the period marked by rapid eye movement, when standard dreams most often occur. According to Stickgold, other studies suggest that the hippocampus isn’t active during REM sleep either. Therefore, he proposes, the brain activity responsible for the Tetris images is probably similar to the dreaming that occurs in REM sleep.

Interpreting REM sleep dreams, however, is a highly subjective process.

“What’s so nice about the images in our experiments is that they are so accurately re-creating the Tetris experience. There’s no interpretation necessary,” Stickgold said.