Consciousness is the awareness of external stimuli and of one's own mental activity.

Consciousness is defined as the state of being awake and aware of one's surroundings such as external objects and one's own existence, such as the ability to feel. It is also associated with such terms as awareness, wakefulness, and sentience. Philosophers and psychologists have studied consciousness since the nineteenth century, with more informal investigations carried out in earlier centuries, such as those of RenéDescartes (1596–1650) in the seventeenth century.

Descartes proposed the concept of thinking about one's own existence, what he called in classical Latin cogito ergo sum, or translated in English “I think, therefore I am.” In reality, humankind probably has been trying to understand consciousness since the brain was able to grasp such a concept.

Study of consciousness since nineteenth century

Investigations of consciousness conducted by German physician and physiologist Wilhelm Wundt (1832–1920), which 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 American psychologist and philosopher William James (1842–1910), who focused on how consciousness helps people adapt to their environment. Behaviorism, pioneered by American psychologist John B. Watson (1878–1958) in the early 1900s, shifted interest from conscious processes to observable behaviors, and the study of consciousness moved 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 the model of human mental functioning conceptualized by Austrian neurologist Sigmund Freud (1856–1939). 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.)

Swiss psychotherapist Carl Jung (1875–1961), Freud's contemporary, 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.

Twenty-first-century studies

In the early part of the twenty-first century, neuroscientists were studying the brain to learn what part of it is directly responsible for consciousness and unconsciousness. Although research was in its early stage, the claustrum, an irregularly shaped part of the brain underneath the inner part of the neocortex, was believed by researchers to control communications between the two hemispheres of the brain, specifically to control attention and consciousness.

American neurologist Mohamad Koubeissi and his colleagues, in an August 2014 article in the journal Epilepsy and Behavior, were able to make a woman unconscious and then conscious again by stimulating her claustrum.

American neuroscientist and psychiatrist Giulio Tononi, of the Department of Psychiatry at the University of Wisconsin at Madison, was developing a potentially important theory on consciousness that he calls integrated information theory. His studies argued that the entire experience of consciousness of any individual is an integration of all the experiences sensed from birth to the present. As such, the brain uses this enormous amount of information to allow the individual to sense the world while conscious. Studies by Tononi and others suggested that this integration of the brain can be measured to show its extent in each individual brain.

Another theory of consciousness was global workspace (GW) theory. Headed by American neuroscientist Bernard Baars of the Neurosciences Institute at La Jolla, California, the theory proposes that all sensations of an individual are sent to a central location, what is called the brain's blackboard, and it is then sent to other locations within the brain to be processed. The sending out of information from the blackboard (memory bank) of the brain represents consciousness in all individuals.

Baars states in his 2005 paper within the journal Progress in Brain Research: “GW theory generates explicit predictions for conscious aspects of perception, emotion, motivation, learning, working memory, voluntary control, and self systems in the brain.” He adds, “Functional brain imaging now shows that conscious cognition is distinctively associated with wide spread of cortical activity, notably toward frontoparietal and medial temporal regions. Unconscious comparison conditions tend to activate only local regions, such as visual projection areas.”

Nature of consciousness

People experience different levels and 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. During REM sleep 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 (hyp-nos), 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 are lack of initiative, selective redistribution of attention, enhanced ability to fantasize, reduced reality testing, and increased suggestibility. In addition, 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.


—A psychological theory of human development that states humans can be conditioned to respond in specific ways to specific stimuli and that given the correct stimuli, behaviors of individuals can be controlled.
—A thin, irregularly shaped sheet of neurons within the brain beneath the inner surface of the neocortex that gathers input from nearly all regions of the cortex and returns this information almost all these regions.
—The ability to experience, sense, and understand subjectively.

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, 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.

See also Descartes, Rene; Sleep .



Churchland, Paul, M. Matter and Consciousness. Cam-bridge: Massachusetts Institute of Technology Press, 2013.

Thompson, Evan. Waking, Dreaming, Being: Self and Consciousness in Neuroscience, Meditation, and Philosophy. New York: Columbia University Press, 2015.


Koubeiss, Mohamad Z, et al. “Electrical Stimulation of a Small Brain Area Reversibly Disrupts Consciousness.” Epilepsy and Behavior 37 (August 2014): 32–35.


Lewis, Tanya. “Scientists Closing in on Theory of Consciousness.” (accessed July 23, 2015).

Neuroscience Training Program, University of Wisconsin– Madison. “Giulio Tonomi.” (accessed July 23, 2015).


Center for Consciousness Studies, Department of Anesthesiology, College of Medicine, University of Arizona, PO Box 245114, Tucson, AR, 85724-5114, (520) 6219317, Fax: (520) 626-6416,, .