<P> A variety of psychoactive drugs, including alcohol, have notable effects on consciousness . These range from a simple dulling of awareness produced by sedatives, to increases in the intensity of sensory qualities produced by stimulants, cannabis, empathogens--entactogens such as MDMA ("Ecstasy"), or most notably by the class of drugs known as psychedelics . 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 . </P> <P> 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 . </P> <P> 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 . Research in this area has not yet reached firm conclusions, but a recent questionnaire - based study identified eleven significant factors contributing to drug - induced states of consciousness: experience of unity; spiritual experience; blissful state; insightfulness; disembodiment; impaired control and cognition; anxiety; complex imagery; elementary imagery; audio - visual synesthesia; and changed meaning of percepts . </P> <P> Phenomenology is a method of inquiry that attempts to examine the structure of consciousness in its own right, putting aside problems regarding the relationship of consciousness to the physical world . This approach was first proposed by the philosopher Edmund Husserl, and later elaborated by other philosophers and scientists . Husserl's original concept gave rise to two distinct lines of inquiry, in philosophy and psychology . In philosophy, phenomenology has largely been devoted to fundamental metaphysical questions, such as the nature of intentionality ("aboutness"). In psychology, phenomenology largely has meant attempting to investigate consciousness using the method of introspection, which means looking into one's own mind and reporting what one observes . This method fell into disrepute in the early twentieth century because of grave doubts about its reliability, but has been rehabilitated to some degree, especially when used in combination with techniques for examining brain activity . </P>

The type of consciousness we experience when we are conscious of external sensations