Why altered states matter for consciousness science

To find what makes the brain conscious, researchers compare conditions in which consciousness is present, absent or changed, while as much else as possible stays the same. Sleep, dreaming, anaesthesia and drug-induced states provide natural contrasts of exactly this kind.

Sleep and dreaming

Established Consciousness fades in deep non-REM sleep and often returns in vivid form during REM sleep, though people woken from non-REM sleep also report dreams surprisingly often. Studies comparing awakenings with and without dream reports have linked dreaming to activity in a posterior “hot zone” of the cortex, a finding that has fed into the debate between major theories.

Lucid dreaming

Established In a lucid dream the sleeper knows they are dreaming. In the late 1970s and early 1980s Stephen LaBerge and colleagues showed this could be verified in the lab: lucid dreamers made pre-agreed eye movements that appeared on recordings during REM sleep. In 2021 an international study (Konkoly et al.) reported two-way communication, with dreamers answering simple maths questions with eye signals. More: the science of lucid dreaming.

Anaesthesia

Established General anaesthesia reliably and reversibly removes consciousness. Different drugs act on different molecular targets, yet they share effects on large-scale brain dynamics, including disrupted communication between brain regions. A measure called the perturbational complexity index, which stimulates the cortex and measures how complex the response is, distinguishes conscious from unconscious states across sleep, anaesthesia and some brain injuries. For the microtubule hypothesis, see this review.

Psychedelic research

Debated Psilocybin, LSD and related compounds act mainly on serotonin 5-HT2A receptors and profoundly alter perception and the sense of self. Brain imaging studies associate them with increased “entropy” or diversity of brain activity and with reduced integrity of the default mode network, the basis of Robin Carhart-Harris’s “entropic brain” hypothesis. Clinical trials of psilocybin for depression have reported encouraging results in controlled settings, but they are small, hard to blind and still under regulatory review. More: psychedelics and consciousness.