Author: Elin Bonyadi
Artist: Bella Marwick
Editor: Leonie Hellwich
From being a means through which our subconscious thoughts are expressed, to being merely a product of random brain activity, people have long theorised about the purpose of dreams. With dreaming being so energetically costly to the brain, it is believed there must be a biologically important reason for it, although this has been difficult to elucidate. However, an exciting new theory has recently been proposed to explain why we dream and why our dreams are so visual. This theory suggests our dreams help protect our visual perception abilities, which might otherwise be lost as we sleep.
The brain has an incredible capacity for adaptation in response to changes in the environment‒a concept known as neural plasticity. Brain regions that usually process a certain type of sensory input, such as visual information, can begin to respond to, or be “taken over” by, other sensory modalities if they no longer receive their usual input. These changes can occur much faster than once thought. A particularly striking study by Merabet and colleagues in 2007 revealed that, after blindfolding normally-sighted individuals while they practised with tactile stimuli, a brain region usually associated with visual processing showed an increased response to tactile stimuli after less than just one hour.
For neuroscientists David Eagleman and Don Vaughn, such findings led to the question: if the brain’s visual regions can adapt so rapidly, why are they not completely taken over during sleep, when the brain is not receiving visual input from the outside world through the eyes?
A new theory by Eagleman and Vaughn in 2021 proposed the reason this complete takeover of the visual regions does not occur overnight is due to dreaming. Their “Defensive Activation Theory” proposes that dreaming is evolution’s way of preserving the visual processing abilities of the brain, by continuing to stimulate the visual regions during sleep. Most dreaming occurs during the REM (rapid eye movement) stage of sleep, characterised by dreams rich in visual content. In REM sleep, signals are sent specifically to the primary visual cortex from a region called the pons. This activation of specifically the primary visual cortex suggests dreams may occur to continue the stimulation of visual regions during sleep. According to the theory, this continued activation ensures these regions are not taken over by other sensory modalities, such as tactile or auditory, which are still active during sleep.
Crucially, this theory would also explain the reduction in the proportion of time spent in REM sleep across the lifespan, as the brain’s capacity for plastic change decreases with age. With reduced plasticity, the threat of the visual areas being taken over is diminished, meaning longer periods of dreaming, and therefore REM sleep, are less necessary. The Defensive Activation Theory also predicts that the more plastic an organism’s brain, the more REM sleep it should have, as the visual regions of these brains would be at greater risk of takeover. In a study in 2021, Eagleman and Vaughn demonstrated evidence seemingly consistent with their theory: they found that greater plasticity was related to a greater proportion of sleep time spent in REM sleep across many primate species. The authors also noted that some medications used to treat depression inhibit dreaming and are frequently linked with visual problems. The authors suggest the reduction or interruption of dreaming caused by these medications may be the cause of these visual problems due to the takeover of the visual regions as a result of less REM sleep and, consequently, less dreaming.
In 2023, Knopper and Hansen published a response to the Defensive Activation Theory. While questioning the evidence the theory is based on due to recent findings that neither plasticity nor REM sleep decline with age, the authors did suggest a brain region which could underlie this mechanism: the locus coeruleus, due to its role in both REM sleep regulation and plasticity. Thus, they recommend further exploration of this region in relation to this theory.
Overall, the Defensive Activation Theory is an exciting new theory seeking to explain why we dream in pictures, a question which has long posed a mystery. However, there is currently a lack of research testing the theory’s validity. Furthermore, the existing evidence by the theory’s authors relies only on directionally consistent trends, which could have alternative explanations, and on indirect measures of visual cortex plasticity. Therefore, more research is needed to establish whether we have finally understood the purpose of dreaming.
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