Why Do We Dream?
Sweet dreams are made of … what? Dreaming is one of the most mysterious aspects of human existance. But why do we dream?
Text by: Pascal Rappard, M.D. | DocCheck Team
We spend about a third of our lives asleep and a significant portion of that time is spent in rapid eye movement (REM) sleep — the stage of sleep associated with vivid and often bizarre dreams. But what is the purpose of dreaming, and why do we need REM sleep?
Sleep And The Visual Cortex
A new theory, proposed by neuroscientists David Eagleman and Don Vaughn, published in Frontiers in Neuroscience, suggests that REM sleep serves a defensive function: it prevents the takeover of the visual cortex by other sensory modalities during the night. The visual cortex is the part of the brain that processes visual information and it occupies a large portion of the cerebral cortex. However, when we are asleep — especially in dark environments — our visual input is drastically reduced. This creates a potential vulnerability for the visual system because other sensory modalities such as touch, hearing or smell, could invade its territory and weaken its connections.
Eagleman and Vaughn argue that REM sleep counteracts this threat by periodically activating the visual system with internally generated images — or dreams. By doing so, REM sleep maintains the integrity and functionality of the visual cortex and preserves its ability to adapt to changing environments. This hypothesis is based on recent findings on neural plasticity, which is the brain’s ability to rewire itself in response to experience. Studies have shown that neural plasticity is very fast and dynamic and that brain regions can lose or gain territory within hours if their inputs are altered. For example, blind people show increased activity and connectivity in their visual cortex when they perform tasks involving touch or hearing, suggesting that these modalities have taken over some of the visual territory.
REM Sleep: Made By Evolution
Eagleman and Vaughn tested their hypothesis by comparing three measures (see below) of plasticity across 25 primate species, including humans. They found that plasticity and REM sleep increased in parallel with evolutionary recency to humans, meaning that more recent species had higher levels of both. They also found that plasticity correlated positively with the proportion of REM sleep across species, meaning that species with more REM sleep had higher plasticity. These findings suggest that REM sleep may be an evolutionary adaptation to enhance plasticity and protect the visual system from sensory competition.
The authors acknowledge that their hypothesis does not exclude other possible functions of REM sleep like learning, memory consolidation, emotional regulation or temperature control. However, they argue that their hypothesis can account for several features of REM sleep that are otherwise puzzling, such as:
- why it is more prevalent in early development, when plasticity is highest;
- why it decreases with aging, when plasticity declines;
- why it is more common in nocturnal animals than diurnal ones;
- and why it is associated with eye movements that mimic visual scanning.
Ultimately, the researchers were not able to answer all questions about dreaming, some aspects will remain a mystery. Nonetheless, the defensive activation theory offers a novel perspective on the origin and function of REM sleep and dreaming. It also raises new questions for future research: How does REM sleep affect visual perception and performance, how does it interact with other factors that influence plasticity, e. g. stress or hormones, and how does it relate to other forms of internally generated imagery like hallucinations or imagination?
Image Source: Ryan Hutton, unsplash
