Neuroscientist David Eagleman has recently shared a new theory about dreaming, based on evolutionary principles rather than conventional neurobiology
Stanford neuroscientist David Eagleman is successfully recognised as being among the celebrated neuroscience communicators of our generation. The last number of years have seen Eagleman hop on the Huberman-esque sexy science bandwagon, where he has not only made neuroscience research accessible and universally interesting by writing with a contagious enthusiasm for the subject, but also reminds both neuroscientists and casual science aficionados how incredible the brain truly is. Eagleman’s most well-known books “The Brain: The Story of You”, “Incognito”, and “Sum”, have all been critically-acclaimed for their philosophical approach to neuroscience and appreciation for novel findings contributing to our knowledge of neural architecture and mechanics. Recently, Eagleman proposed the novel mind-boggling Defence Activation Theory which suggests, in his words, “what dreaming (has) to do with the rotation of the planet”. Eagleman and his student, Vaughn, published the theory in 2021 and have also described it in true modern-day neuroscientist fashion on his “Inner Cosmos” podcast.
Considering that as humans, we spend a third of our lives asleep, we have an astonishingly ignorant understanding of dreams and their evolutionary underpinnings. While it’s possible that when you think of dreams, a certain uncomfortable Freudian theory comes to mind, it is now recognised that Freud’s fascination with consciousness was special and has led to an overdue study of the phenomenology of sleep and its corresponding neurobiological activity. Nowadays, we can appreciate the role of sleep in memory consolidation, physiological restoration, and facilitating the glymphatic system for waste product removal. Moreover, the development of neuroimaging technologies allows for a more detailed understanding of the neural mechanisms behind dreaming, revealing links between wakefulness and REM sleep activity, the period during which dreams are likely to occur. Further research demonstrated activation of the visual system and visual processing regions during REM sleep, where it was postulated that the so-called “Pontogeniculooccipital waves” (PGO waves) serve a role in brain development, informing the visual system on eye movements, and facilitating responses to novel stimuli. Eagleman has taken this previously established research on visual-system activation during REM sleep and dreaming, and studied the concept through an evolutionary lens.
Eagleman introduces his Defence Activation Theory with a synthesis of extensive research examples of neuroplasticity, i.e. the gain or loss of neural real estate due to a significant reduction or increase in sensory input. Previous studies have demonstrated a shift in the neural architecture of visually-impaired participants, such as increased areas dedicated to audition and somatosensation as a result of a disengaged visual system. So, how does our visual system maintain its acuity, given that we experience complete visual deprivation at night every 12 hours or so? Interestingly, the loss of light solely inhibits visual input, while we maintain our ability to smell, touch, and hear during sleep. Eagleman delves into the neurophysiology of REM sleep and dreaming, with research involving paralysis of major muscle groups alongside PGO wave activity to simulate a visual experience while our eyes remain closed. The Defence Activation Theory proposes that this process renders an evolutionary defence mechanism adopted by the visual cortex to uphold the neural connections dedicated to vision.
The Defence Activation Theory is best appreciated as an evolutionary concept, as opposed to a robust scientifically-proven hypothesis (as suggested by Eagleman himself). However, an abundance of research across species supports his proposition. To make these inferences, Eagleman focused his attention on plasticity during various stages of the sleep cycle, taking coordinated REM sleep activity and phylogenic complexity to be indicators that the plasticity of a system requires a corresponding amount of neural activity for maintenance. Furthermore, the theory is supported by the correlation between decreasing REM sleep and decreasing plasticity with age. Initially, it was only the dormant visual cortices of elephants during REM sleep that led researchers to question the theory, until it was established that elephants typically only sleep for 2 hours per night. Alongside their sharp night vision, this means their neural cortices aren’t competing in the same way, and that they receive sufficient external sensory input for the visual system during wakefulness. However, limitations such as solely using neuroimaging data must be considered, and the hypothesis should be further examined in the context of occipital lobe activation, through research such as administration of REM-reducing medication.
Overall, Eagleman’s evolutionary perspective provides a new angle from which we can consider the neuroscience of dreams, leading many to wonder about the implications of the Defence Activation Theory in appreciating the human connectome. Oftentimes, our own understanding of topics is impaired by an insufficient level of detail, combined with the influence of previous research that biases our own. Eagleman’s work provides a lesson, perhaps not solely applicable to neuroscience, to zoom out and view research from a different perspective, in a different context, and its place within the “bigger picture”.
Photo by Megan Thomas on Unsplash.
This article was written by Amy Elliott and edited by Julia Dabrowska. Interested in writing for WiN UK yourself? Contact us through the blog page and the editors will be in touch!
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