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See Inside November / December 2010

Dream States: A Peek into Consciousness

Although we rarely remember our nighttime reveries, they may hold the key to consciousness

Further evidence comes from the study of brain-damaged patients by neuropsychoanalyst Mark Solms of the University of Cape Town in South Africa. When a part of the brain stem known as the pons is destroyed, people no longer experience REM sleep. But only one in 26 of such patients reports a loss of dreaming, and nobody has ever reported loss of dreaming from limited pons damage.

The regions critical for dreaming are not in the pons. They include the visual and audiovisual regions in and near the temporoparietal-occipital junction in the neocortex. Destruction of small portions of these areas leads to the loss of specific dreaming dimensions. For example, a stroke, tumor or other calamity in the cortical region necessary for color or motion perception will leach hue or movement from dreams.

Moreover, medications that manipulate dopamine levels strongly affect dreaming while leaving the REM sleep cycle unaffected. L-dopa, the most popular medication for Parkinson’s disease, increases the frequency and vividness of dreams, whereas antipsychotic drugs that block dopamine reduce dreaming.

The dissociation of dreaming from REM sleep serves as a conceptual clearing of the deck for neuroscientists such as myself. Now we can focus on the neuronal causes of conscious mental activity, whether in a dreaming or wakeful state, without being confused by extraneous factors such as REM or non-REM sleep that, it turns out, do not pertain to subjective experience per se.

The Mind-Body Problem
Why am I so confident I experience anything while dreaming? Maybe I am unconscious while slumbering and confabulate my dreams when I awaken.

This is unlikely for many reasons. The bizarreness and vividness of dreams are distinct from normal experience and therefore unlikely to be “retrofitted.” Indeed, people with memory deficits do not report fewer dreams. Additionally, the length of dream reports correlates well with time elapsed in REM dreams.

More evidence comes from people with REM sleep behavior disorder, who lack the muscle paralysis, known as atonia, typical of REM sleep. They act out their dreams, sometimes even harming themselves or bed partners, and their actions match their dream reports. They might, for instance, move their legs while asleep and later report that they dreamed of walking.

Dreams are of great interest to the student of the mind-body problem, because they bear witness that the brain alone is sufficient to generate consciousness. We dream with eyes shut in the dark, disconnected from the outside world. The brain regions responsible for basic sensory perception are deactivated. Nor is behavior necessary, as we are motionless except for our breathing and eye movements. Thus, dreaming supports the old philosophical brain-in-the-vat idea that saw its modern renaissance in The Matrix.

Cognitive neuroscientists have recently learned to decode some simple mental states—in essence, a primitive form of mind reading. When scientists ask people to view one of two images—a portrait or a photograph of a house—or to imagine either a face or a house, they can tell from brain analyses which of the two the person is seeing or imagining.

Once such techniques become more sophisticated, they could be put to use in dream work, so that in addition to studying the physiology of the dreaming brain, investigators will be able to read out the content of the dream itself. Then neuroscience will be in a much better position to answer the age-old questions that have fascinated everyone from oracles and shamans to Freud and, more recently, science-fiction enthusiasts: Why do we dream, and what do dreams mean?

This article was originally published with the title "Consciousness Redux: Dream States."

(Further Reading)

  • Dreaming and the Brain: Toward a Cognitive Neuroscience of Conscious States. J. Allan Hobson, Edward F. Pace-Schott and Robert Stickgold in Behavioral and Brain Sciences, Vol. 23, pages 793–842; 2000.
  • Dreaming and REM Sleep Are Controlled by Different Brain Mechanisms. Mark Solms in Behavioral and Brain Sciences, Vol. 23, pages 843–850; 2000.
  • Dreaming and the Brain: From Phenomenology to Neurophysiology. Yuval Nir and Giulio Tononi in Trends in Cognitive Sciences, Vol. 14, pages 88–100; 2010.
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