LSD, mescaline, psilocybin, dimethyltryptamine, and others in this group can produce major distortions of perception, including hallucinations; some users even describe their drug-induced experiences as mystical or spiritual in quality. The brain mechanisms underlying these effects are not as well understood as those induced by use of alcohol, but there is substantial evidence that alterations in the brain system that uses the chemical neurotransmitter serotonin play an essential role.There has been some research into physiological changes in yogis and people who practise various techniques of meditation. Some research with brain waves during meditation has reported differences between those corresponding to ordinary relaxation and those corresponding to meditation. It has been disputed, however, whether there is enough evidence to count these as physiologically distinct states of consciousness.The most extensive study of the characteristics of altered states of consciousness was made by psychologist Charles Tart in the 1960s and 1970s. Tart analyzed a state of consciousness as made up of a number of component processes, including exteroception (sensing the external world); interoception (sensing the body); input-processing (seeing meaning); emotions; memory; time sense; sense of identity; evaluation and cognitive processing; motor output; and interaction with the environment. Each of these, in his view, could be altered in multiple ways by drugs or other manipulations. The components that Tart identified have not, however, been validated by empirical studies.
The primary focus is on understanding what it means biologically and psychologically for information to be present in consciousness—that is, on determining the neural and psychological correlates of consciousness. In medicine, consciousness is assessed by observing a patients arousal and responsiveness, and can be seen as a continuum of states ranging from full alertness and comprehension, through disorientation, delirium, loss of meaningful communication, and finally loss of movement in response to painful stimuli. Issues of practical concern include how the presence of consciousness can be assessed in severely ill, comatose, or anesthetized people, and how to treat conditions in which consciousness is impaired or disrupted. The degree of consciousness is measured by standardized behavior observation scales such as the Glasgow Coma Scale. Etymology
In the late 20th century, philosophers like Hamlyn, Rorty, and Wilkes have disagreed with Kahn, Hardie and Modrak as to whether Aristotle even had a concept of consciousness. Aristotle does not use any single word or terminology to name the phenomenon; it is used only much later, especially by John Locke. Caston contends that for Aristotle, perceptual awareness was somewhat the same as what modern philosophers call consciousness.The origin of the modern concept of consciousness is often attributed to Lockes Essay Concerning Human Understanding, published in 1690. Locke defined consciousness as "the perception of what passes in a mans own mind". His essay influenced the 18th-century view of consciousness, and his definition appeared in Samuel Johnsons celebrated Dictionary (1755).