Dreams have long been a source of intrigue and mystery, leading to numerous theories about their purpose. Among these, a compelling hypothesis suggests that dreams play a defensive role for the brain’s visual cortex. This idea, proposed by neuroscientist David Eagleman of Stanford University, centers on the concept of neuroplasticity—the brain’s ability to adapt and reshape itself.
Eagleman’s hypothesis hinges on the notion of neural competition for survival and territory within the brain. He explains that the brain’s various sensory areas engage in a fierce battle for neural territory, especially when sensory inputs change. This phenomenon is evident in individuals who have undergone drastic brain changes. For example, children who have lost half of their brain due to health issues can still regain normal functions, as the remaining half reorganizes and compensates for the loss. Similarly, loss of a sense like sight or hearing often leads to heightened sensitivity in other senses, indicating a takeover of the brain region previously dedicated to the lost sense.
This reorganization can occur rapidly. Research by Lotfi Merabet of Harvard Medical School demonstrated that the takeover of idle brain areas by other senses can begin in as little as 45 minutes. However, this dynamic changes during sleep. While we can smell, hear, and feel during sleep, visual input is absent, except during REM (Rapid Eye Movement) sleep. REM sleep, which begins about 90 minutes after falling asleep, involves two key processes: muscle paralysis and activation of the visual cortex.
Eagleman posits that REM sleep serves to protect the visual cortex from being overtaken by other sensory processes. During REM sleep, the brain’s activity is primarily centered in the visual cortex. This activity acts as a defense mechanism, ensuring that the neurons in the visual cortex stay active in their primary function—processing visual information—thus preventing takeover by neurons processing other sensory information.
The requirement for REM sleep varies with the plasticity of the brain. Newborns, with highly adaptable brains, spend nearly half of their sleep time in REM. In contrast, adults, whose brains are less flexible, spend less time in REM sleep. This correlation is observed across different species, suggesting that less hardwired brains at birth require more REM sleep.
While Eagleman’s theory has its supporters, it also faces skepticism from some dream researchers. Critics like Antonio Zadra of the University of Montreal argue that the theory oversimplifies dreams and their relationship with REM sleep. However, Deirdre Leigh Barrett, a psychologist at Harvard University, finds merit in the idea, particularly in the correlation between complex brains and elaborate dreams.
Eagleman acknowledges that his theory may not be the sole explanation for dreams and REM sleep. He likens dreaming to a computer screen saver that activates every 90 minutes, not to prevent image freezing but to safeguard the visual cortex from being taken over by other brain functions. This process, he suggests, allows us to maintain our visual capabilities during our waking hours.
