ABSTRACT
Altered states of consciousness (ASCs) are surprisingly hard to define or measure. Neither subjective methods, like verbal reports, nor objective measures, such as how the state is induced, or physiological and behavioural criteria, are entirely satisfactory. Attempts to map ASCs are reviewed, including two- or three-dimensional maps such as Tart’s and Laureys’, and Hobson’s AIM model, but the true space of possibilities is likely to be vast. Psychoactive drugs include stimulants (amphetamine, MDMA), anaesthetics (nitrous oxide, ketamine), cannabis, and major psychedelics (mescaline, psilocybin, DMT, ayahuasca, LSD). Experiments using these drugs were long prohibited, but brain scans are beginning to reveal changes to attention systems and the default mode network, and clinical studies show promise in alleviating depression and anxiety, especially for the terminally ill. Meditation can also lead to ASCs: for example, the ‘jhanas’ are eight discrete states induced by deep concentrative meditation. Consideration of altered states in mental illness, such as anorexia or depression, raises further problems in deciding what counts as an altered state and what to compare it with. Is there a ‘normal’ or baseline state for comparison? Many of this chapter’s questions apply also to hypnosis, and we briefly review the ongoing state/non-state debate.
