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Awareness of external stimuli and of one's own mental activity.

Wilhelm Wundt's investigations of consciousness, begun in 1879, were central to the development of psychology as a field of study. Wundt's approach, called structuralism, sought to determine the structure of consciousness by recording the verbal descriptions provided by laboratory subjects to various stimuli, a method that became known as introspection. The next major approach to the study of consciousness was the functionalism of William James, who focused on how consciousness helps people adapt to their environment. Behaviorism, pioneered by John B. Watson in the early 1900s, shifted interest from conscious processes to observable behaviors, and the study of consciousness faded into the background for almost half a century, especially in the United States, until it was revived by the "cognitive revolution" that began in the 1950s and 1960s.

The existence of different levels of consciousness was at the heart of Sigmund Freud's model of human mental functioning. In addition to the conscious level, consisting of thoughts and feelings of which one is aware, Freud proposed the existence of the unconscious, a repository for thoughts and feelings that are repressed because they are painful or unacceptable to the conscious mind for some other reason. He also formulated the concept of the preconscious, which functions as an intermediate or transitional level of mind between the unconscious and the conscious. A preconscious thought can quickly become conscious by receiving attention, and a conscious thought can slip into the preconscious when attention is withdrawn from it. In contrast, the repressed material contained in the unconscious can only be retrieved through some special technique, such as hypnosis or dream interpretation. (What Freud called the unconscious is today referred to by many psychologists as the subconscious.) Freud's contemporary, Carl Jung, posited the existence of a collective unconscious shared by all people which gathers together the experiences of previous generations. The collective unconscious contains images and symbols, called archetypes, that Jung found are shared by people of diverse cultures and tend to emerge in dreams, myths, and other forms. In Jung's view, a thorough analysis of both the personal and collective unconscious was necessary to fully understand the individual personality.

People experience not only different levels, but also different states of consciousness, ranging from wakefulness (which may be either active or passive) to deep sleep. Although sleep suspends the voluntary exercise of both bodily functions and consciousness, it is a much more active state than was once thought. Tracking brain waves with the aid of electroencephalograms (EEGs), researchers have identified six stages of sleep (including a pre-sleep stage), each characterized by distinctive brain-wave frequencies. In rapid eye movement (REM) sleep, which makes up 20% of sleep time, the same fast-frequency, low-amplitude beta waves that characterize waking states occur, and a person's physiological signs—heart rate, breathing, and blood pressure—also resemble those of a waking state. It is during REM sleep that dreams are experienced. Delta waves demarcate the deepest levels of sleep, when heart rate, respiration, temperature, and blood flow to the brain are reduced and growth hormone is secreted.

Certain waking states, which are accompanied by marked changes in mental processes, are considered states of altered consciousness. One of these is hypnosis, a highly responsive state induced by a hypnotist through the use of special techniques. While the term "hypnosis" comes from the Greek word for sleep (hypnos), hypnotized people are not really asleep. Their condition resembles sleep in that they are relaxed and out of touch with ordinary environmental demands, but their minds remain active and conscious. Other characteristics of hypnosis include lack of initiative, selective redistribution of attention, enhanced ability to fantasize, reduced reality testing, and increased suggestibility. Also, hypnosis is often followed by post-hypnotic amnesia, in which the person is unable to remember what happened during the hypnotic session. Hypnosis has proven useful in preventing or controlling various types of pain, including pain from dental work, childbirth, burns, arthritis, nerve damage, and migraine headaches.

In meditation, an altered state of consciousness is achieved by performing certain rituals and exercises. Typical characteristics of the meditative state include intensified perception, an altered sense of time, decreased distraction from external stimuli, and a sense that the experience is pleasurable and rewarding. While meditation is traditionally associated with Zen Buddhism, a secular form called Transcendental Meditation (TM) has been widely used in the United States for purposes of relaxation. It has been found that during this type of meditation, people consume less oxygen, eliminate less carbon dioxide, and breathe more slowly than when they are in an ordinary resting state.

Consciousness may be altered in a dramatic fashion by the use of psychoactive drugs, which affect the interaction of neurotransmitters and receptors in the brain. They include illegal "street drugs," tranquilizers and other prescription medications, and such familiar substances as alcohol, tobacco, and coffee. The major categories of psychoactive drugs include depressants, which reduce activity of the central nervous system; sedatives, another type of depressant that includes barbiturates such as Seconal and Nembutal; anxiolytics (traditionally referred to as tranquilizers); narcotics—including heroin and its derivatives—which are addictive drugs that cause both drowsiness and euphoria, and are also pain-killers; psychostimulants, such as amphetamines and cocaine, which stimulate alertness, increase excitability, and elevate moods; and psychedelics or hallucinogens, such as marijuana and LSD. Psychedelics, which affect moods, thought, memory, and perception, are particularly known for their consciousness-altering properties. They can produce distortion of one's body image, loss of identity, dreamlike fantasies, and hallucinations. LSD (lysergic acid diethylamide), one of the most powerful psychedelic drugs, can cause hallucinations in which time is distorted, sounds produce visual sensations, and an out-of-body feeling is experienced.

Various states of consciousness are viewed differently by different cultures and even subcultures. In the United States, for example, hallucinations are devalued by mainstream culture as a bizarre sign of insanity, whereas the youth counterculture of the 1960s viewed drug-induced hallucinations as enlightening, "mind-expanding" experiences. In certain other societies, hallucinations are respected as an important therapeutic tool used by ritual healers.

Further Reading

Dennett, D.C. Brainstorms. Cambridge, MA: Bradford Books, 1980.

Freud, Sigmund. "The Unconscious." In The Standard Edition of the Complete Psychological Works of Sigmund Freud. London: Hogarth Press, 1962.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Abacus to Courage