Theory why we dream

Ben clearly benefited from the redistribution of his visual cortex to other senses because he had permanently lost his eyes, but what about the participants in the blindfold experiments? If our loss of a sense is only temporary, then the rapid conquest of brain territory may not be so helpful. Of course, this refers to the vast majority of evolutionary time, not to our present electrified world.

Our ancestors effectively were unwitting participants in the blindfold experiment, every night of their entire lives.

We suggest that the brain preserves the territory of the visual cortex by keeping it active at night. In this view, dreams are primarily visual precisely because this is the only sense that is disadvantaged by darkness. Thus, only the visual cortex is vulnerable in a way that warrants internally-generated activity to preserve its territory. In humans, sleep is punctuated by rapid eye movement REM sleep every 90 minutes. This is when most dreaming occurs.

Although some forms of dreaming can occur during non-REM sleep, such dreams are abstract and lack the visual vividness of REM dreams. This activity in the visual cortex is presumably why dreams are pictorial and filmic. The dream-stoking circuitry also paralyzes your muscles during REM sleep so that your brain can simulate a visual experience without moving the body at the same time.

The anatomical precision of these circuits suggests that dream sleep is biologically important—such precise and universal circuitry rarely evolves without an important function behind it.

The defensive activation theory makes some clear predictions about dreaming. For example, because brain flexibility diminishes with age, the fraction of sleep spent in REM should also decrease across the lifespan.

REM sleep appears to become less necessary as the brain becomes less flexible. Of course, this relationship is not sufficient to prove the defensive activation theory. To test it on a deeper level, we broadened our investigation to animals other than humans.

How might we measure this? We looked at the time it takes animals of each species to develop. How long do they take to wean from their mothers? How quickly do they learn to walk? How many years until they reach adolescence? As predicted, we found that species with more flexible brains spend more time in REM sleep each night. Although these two measures—brain flexibility and REM sleep—would seem at first to be unrelated, they are in fact linked.

As a side note, two of the primate species we looked at were nocturnal. But this does not change the hypothesis: whenever an animal sleeps, whether at night or during the day, the visual cortex is at risk of takeover by the other senses. Nocturnal primates, equipped with strong night vision, employ their vision throughout the night as they seek food and avoid predation.

When they subsequently sleep during the day, their closed eyes allow no visual input, and thus, their visual cortex requires defense. Dream circuitry is so fundamentally important that it is found even in people who are born blind.

This is because other senses have taken over their visual cortex. Plenty of mammals and birds dream, too. Lots of theories have been offered: dreams are used to regulate emotion, like dealing with fears; to consolidate memory, replaying things from your day to help remember them; to solve, or on the other hand to forget, real-world problems. Another theory suggests they help the brain predict its own future states.

In his research, Hoel works with artificial neural networks—machine learning. Think of Deep Mind, the Google artificial intelligence program that beat the best human players at the almost infinitely complex Japanese strategy game Go.

To prevent that, programmers often introduce random variables, or noise in the data. Have you ever had a problem that just seemed to defy solution? You think and think, but you remain stuck. Then you go to bed, wake up the next morning, and presto, the solution appears. It might well be, Hoel would say, that your thinking was overfitted for the task—just like a machine learning program in need of disruption.

When he was young, he loved reading. His mother ran the bookstore Jabberwocky in Newburyport, Mass. He always wanted to be a writer, but ended up studying cognitive science at Hampshire College, and went on to get a Ph. He did become a fiction writer, too: his novel The Revelations will be published by the Overlook Press in early April. Harry Potter never went there. Fiction has all sorts of purposes—aesthetic, emotional, even political—but probably also has an evolutionary role, Hoel says.

The theory of why we dream that considers neuronal activation, stimulation, and state of wakefulness is known as the Activation-Input-Mode AIM. The AIM theory is one of the more popular biological theories for dreaming. According to AIM, activation involves the amount of neural activation and ranges from low to high activation. The input level of consciousness is the extent to which stimulation is external or internal, and the mode consists of dimensions that range from wakeful to dreaming states.

We will learn more about this in later sessions.