A psychopathologist investigates the possibility that our other states of consciousness are as real as our waking states. It is based on the novel of the same name by playwright and screenwriter Paddy Chayefsky. The film was adapted from Chayefsky's only novel and is his final screenplay. Both the novel and the film are based in part on John C. Lilly's sensory deprivation research conducted in isolation tanks under the influence of psychoactive drugs like mescaline, ketamine, and LSD.

References:
https://en.wikipedia.org/wiki/Altered_States