Understanding the Brain


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Representation of consciousness from the seventeenth century.

Consciousness is a term that has been used to refer to a variety of aspects of the relationship between the mind and the world with which it interacts.[1] It has been defined, at one time or another, as: subjective experience; awareness; the ability to experience feelings; wakefulness; having a sense of selfhood; or as the executive control system of the mind.[2] Despite the difficulty of definition, many philosophers believe that there is a basic underlying intuition about consciousness that is shared by nearly all people.[3] As Max Velmans and Susan Schneider wrote in The Blackwell Companion to Consciousness:

"Anything that we are aware of at a given moment forms part of our consciousness, making conscious experience at once the most familiar and most mysterious aspect of our lives."[4]

In philosophy, consciousness is often said to imply four characteristics: subjectivity, change, continuity, and selectivity.[2][5] Philosopher Franz Brentano has also suggested intentionality or aboutness (that consciousness is about something); however, there is no consensus on whether intentionality is a requirement for consciousness.[6]

Issues of practical concern in the philosophy of consciousness include whether consciousness can ever be explained mechanistically; whether non-human consciousness exists and if so how it can be recognized; at what point in fetal development consciousness begins; and whether it may ever be possible for computers to achieve a conscious state.[7][8][9]

At one time consciousness was viewed with skepticism by many scientists and considered within the domain of philosophers and theologians, but in recent years it has been an increasingly significant topic of scientific research. In psychology and neuroscience, the focus of most research 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. The majority of experimental studies use human subjects and assess consciousness by asking subjects for a verbal report of their experiences (e.g., "tell me if you notice anything when I do this"). Issues of interest include phenomena such as subliminal perception, blindsight, denial of impairment, and altered states of consciousness produced by psychoactive drugs or spiritual or meditative techniques.

In medicine, consciousness is assessed by observing a patient's arousal and responsiveness, and can be seen as a continuum of states ranging from full alertness and comprehension, through disorientation, then delirium, then loss of any meaningful communication, and ending with loss of movement in response to painful stimuli.[10] 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.[11]

Etymology and early history

The origin of the modern concept of consciousness is often attributed to John Locke's Essay Concerning Human Understanding, published in 1690.[12] Locke explicitly defined consciousness as “the perception of what passes in a man’s own mind.”[13] His essay had much influence on the 18th century view of consciousness, and his definition appeared in Samuel Johnson's celebrated Dictionary (1755).

The earliest English language uses of "conscious" and "consciousness" date back, however, to the 1500s. The English word "conscious" originally derived from the Latin conscius (con- "together" + scire "to know"), but the Latin word did not have the same meaning as our word — it meant knowing with, in other words having joint or common knowledge with another.[14] There were, however, many occurrences in Latin writings of the phrase conscius sibi, which translates literally as "knowing with oneself", or in other words sharing knowledge with oneself about something. Taken literally this is nonsense, but it had the figurative meaning of knowing that one knows, as the modern English word "conscious" does. In its earliest uses in the 1500s, the English word retained the Latin meaning. For example Thomas Hobbes in Leviathan wrote: "Where two, or more men, know of one and the same fact, they are said to be Conscious of it one to another." The Latin conscius sibi was rendered in English as "conscious to oneself" or "conscious unto oneself". For example, Archbishop Ussher wrote in 1613 of "being so conscious unto myself of my great weakness".[15]

A related word was conscientia, which primarily means moral conscience. In the literal sense, "conscientia" means knowledge-with, that is, shared knowledge. The word first appears in Latin juridical texts by writers such as Cicero.[16] Here, conscientia is the knowledge that a witness has of the deed of someone else.[17] René Descartes (1596–1650) is generally taken to be the first philosopher to use "conscientia" in a way that does not fit this traditional meaning.[citation needed] Descartes used "conscientia" the way modern speakers would use "conscience."[18] In Search after Truth he says "conscience or internal testimony" (conscientia vel interno testimonio).[19]

Shortly thereafter, in Britain, the neo-Platonist theologian Ralph Cudworth used something resembling the modern meaning of consciousness in his "True Intellectual System of the Universe" (1678), although he never explicitly defined the term.[20]

Philosophical approaches

The philosophy of mind has given rise to many stances regarding consciousness. They differ in the answers they give to a set of fundamental questions, including:

  1. Is consciousness a valid concept or a conceptual error?
  2. Is it a single unified entity or a collection of distinct entities?
  3. How does it relate to language?
  4. Can it be explained in terms of the laws of physics?
  5. Why are we convinced that other people (or even we ourselves) possess consciousness?
  6. Why do we believe that some animals possess consciousness, and is there any way to test this belief?
  7. What is the nature of experience, and particularly what is the nature of sensory qualities such as the color red?

The question about the relationship between consciousness and the physical realm is perhaps the most contentious of all: several schools of thought are defined mainly in terms of the answers they give to it.

Is it a valid concept?

A majority of philosophers have felt that the word consciousness names a genuine entity, but some who belong to the physicalist and behaviorist schools have not been convinced; many scientists have also been dubious. The most compelling argument in favor is that the vast majority of mankind has an overwhelming intuition that there truly is such a thing. The argument against is that this intuition, however compelling it may be, is false.[citation needed] Gilbert Ryle, for example, argued that traditional understanding of consciousness depends on a Cartesian dualist outlook that divides into mind and body, mind and world. He proposed that we speak not of minds, bodies, and the world, but of individuals, or persons, acting in the world. Thus, by speaking of 'consciousness,' we end up misleading ourselves by thinking that there is any sort of thing as consciousness separated from behavioral and linguistic understandings.[citation needed]

Another problem that concerns many philosophers and scientists is the difficulty of producing a definition that does not rely on circularity or fuzziness. The neuroscientist Antonio Damasio, for example, calls consciousness "the feeling of what happens", and defines it as "an organism's awareness of its own self and its surroundings".[21] These formulations seem intuitively reasonable, but they are difficult to apply to specific situations.

Is it a single thing?

Many philosophers have argued that consciousness is a unitary concept that is understood intuitively by the majority of people in spite of the difficulty in defining it. Others, though, have argued that the level of disagreement about the meaning of the word indicates that it is an umbrella term meaning different things to different people.

Ned Block proposed a distinction between two types of consciousness that he called phenomenal (P-consciousness) and access (A-consciousness).[22] P-consciousness, according to Block, is simply raw experience: it is moving, colored forms, sounds, sensations, emotions and feelings with our bodies and responses at the center. These experiences, considered independently of any impact on behavior, are called qualia. A-consciousness, on the other hand, is the phenomenon whereby information in our minds is accessible for verbal report, reasoning, and the control of behavior. So, when we perceive, information about what we perceive is access conscious; when we introspect, information about our thoughts is access conscious; when we remember, information about the past is access conscious, and so on. Although some philosophers, such as Daniel Dennett, have disputed the validity of this distinction,[23] others have broadly accepted it. David Chalmers has argued that A-consciousness can in principle be understood in mechanistic terms, but that understanding P-consciousness is much more challenging: he calls this the hard problem of consciousness. Dennett denies that the concept of qualia is coherent, that P-consciousness is intrinsically different from A-consciousness, and that there is anything especially hard about the "hard problem".

Berit Brogaard has argued that even if there is a single concept of consciousness, it does not have determinate application conditions. So, there are cases in which it is neither the case that a state is conscious nor the case that it is not conscious.

How does it relate to the physical world?

The first influential philosopher to discuss this question specifically was Descartes, and the answer he proposed is known as Cartesian dualism. Descartes proposed that consciousness resides within an immaterial domain he called res cogitans (the realm of thought), in contrast to the domain of material things which he called res extensa (the realm of extension).[24] He suggested that the interaction between these two domains occurs inside the brain, perhaps in a small midline structure called the pineal gland.[25]

Although it is widely accepted that Descartes explained the problem very cogently, few later philosophers have been happy with his solution, and in particular his emphasis on the pineal gland has generally been ridiculed. Alternative solutions, however, have been extremely diverse. They can be divided broadly into two categories: dualist solutions that maintain Descartes's rigid distinction between the realm of consciousness and the realm of matter but give different answers for how the two realms relate to each other; and monist solutions that maintain that there is really only one realm of being, of which consciousness and matter are both aspects. Each of these categories itself contains numerous variants. The two main types of dualism are substance dualism (which holds that the mind is formed of a distinct type of substance not governed by the laws of physics) and property dualism (which holds that the laws of physics are universally valid but cannot be used to explain the mind). The three main types of monism are physicalism (which holds that the mind consists of matter organized in a particular way), idealism (which holds that only thought truly exists and matter is merely an illusion), and neutral monism (which holds that both mind and matter are aspects of a distinct essence that is itself identical to neither of them). There are also, however, a large number of idiosyncratic theories that cannot cleanly be assigned to any of these camps.

Some theorists hold that phenomenal consciousness in particular creates an explanatory gap. Colin McGinn takes the New Mysterianism position that it can't be solved, and David Chalmers criticizes purely physical accounts of mental experiences based on the idea that philosophical zombies are logically possible and supports property dualism.

How does it relate to language?

In humans, the clearest visible indication of consciousness is the ability to use language. Medical assessments of consciousness rely heavily on an ability to respond to questions and commands, and in scientific studies of consciousness, the usual criterion for awareness is verbal report (that is, subjects are deemed to be aware if they say that they are). Thus there is a strong connection between consciousness and language. Philosophers differ, however, on whether language is essential to consciousness or merely the most powerful tool for assessing it.

Descartes believed that language is essential to consciousness. Thus, he argued that animals lack consciousness because they lack language. Others have reached the same conclusion, though sometimes for different reasons. Julian Jaynes argued in The Origin of Consciousness in the Breakdown of the Bicameral Mind that for consciousness to arise, language needs to have reached a fairly high level of complexity. Merlin Donald also argued for a critical dependence of consciousness on the ability to use symbols in a sophisticated way.[26]

Those are, however, minority views. If language is essential, then speechless humans (infants, feral children, aphasics, etc.) could not be said to be conscious, a conclusion that the majority of philosophers have resisted. The implication that humans, and not animals, are conscious is also widely resisted by theorists such as evolutionary psychologists; as well as animal rights activists. [27]

Why do people believe that other people are conscious?

Many philosophers consider experience to be the essence of consciousness, and believe that experience can only fully be known from the inside, subjectively. But if consciousness is subjective and not visible from the outside, why do the vast majority of people believe that other people are conscious, but rocks and trees are not?[28] This is the problem of other minds. It is particularly acute for people who believe in the possibility of philosophical zombies, that is, people who think it is possible in principle to have an entity that is physically indistinguishable from a human being and behaves like a human being in every way but nevertheless lacks consciousness.

The most commonly given answer is that we attribute consciousness to other people because we see that they resemble us in appearance and behavior: we reason that if they look like us and act like us, they must be like us in other ways, including having experiences of the sort that we do. There are, however, a variety of problems with that explanation. For one thing, it seems to violate the principle of parsimony, by postulating an invisible entity that is not necessary to explain what we observe. Some philosophers, such as Daniel Dennett in an essay titled The Unimaginable Preposterousness of Zombies, argue that people who give this explanation do not really understand what they are saying. More broadly, philosophers who do not accept the possibility of zombies generally believe that consciousness is reflected in behavior (including verbal behavior), and that we attribute consciousness on the basis of behavior. A more straightforward way of saying this is that we attribute experiences to people because they tell us about their experiences.

Scientific approaches

For many decades, consciousness as a research topic was avoided by the majority of mainstream scientists, because of a general feeling that a phenomenon defined in subjective terms could not properly be studied using objective experimental methods.[29] Even so, some research on topics associated with consciousness was conducted under the banner of attention. Starting in the 1980s, an expanding community of neuroscientists and psychologists have associated themselves with a field called Consciousness Studies, giving rise to a stream of experimental work published in journals such as Consciousness and Cognition, and methodological work published in journals such as the Journal of Consciousness Studies, along with regular conferences organized by groups such as the Association for the Scientific Study of Consciousness.

Broadly speaking, scientific approaches are based on two core concepts. The first identifies the content of consciousness with the experiences that are reported by human subjects; the second makes use of the concept of consciousness that has been developed by neurologists and other medical professionals who deal with patients whose behavior is impaired in a variety of ways. In either case, the ultimate goals are to develop techniques for assessing consciousness objectively in humans as well as other animals, and to understand the neural and psychological mechanisms that underlie it.

Cognitive psychology and cognitive neuroscience

Modern scientific investigations into consciousness are based on psychological statistical studies and case studies of consciousness states and the deficits caused by surgery, trauma or illness that disrupt the normal functioning of human senses and cognition. Another approach is experimental work on unconscious perception, e.g., the investigation of priming effects using subliminal stimuli. These discoveries suggest that the mind is a complex structure derived from various localized functions that are bound together with a unitary awareness.[citation needed]

Several studies point to common mechanisms in different clinical conditions that lead to loss of consciousness. Persistent vegetative state (PVS) is a condition in which an individual loses the higher cerebral powers of the brain, but maintains sleep-wake cycles with full or partial autonomic functions. Studies comparing PVS with healthy, awake subjects consistently demonstrate an impaired connectivity between the deeper (brainstem and thalamic) and the upper (cortical) areas of the brain. In addition, it is agreed that the general brain activity in the cortex is lower in the PVS state. Some electroneurobiological interpretations of consciousness characterize this loss of consciousness as a loss of the ability to resolve time (similar to playing an old phonographic record at very slow or very rapid speed), along a continuum that starts with inattention, continues on sleep, and arrives to coma and death .[30] It is likely that different components of consciousness can be teased apart with anesthetics, sedatives and hypnotics. These drugs appear to act differently on several brain areas to disrupt, to varying degrees, different components of consciousness. The ability to recall information, for example, may be disrupted by anesthetics acting on the hippocampal cortex. Neurons in this region are particularly sensitive to anesthetics at the time loss of recall occurs. Direct anesthetic actions on hippocampal neurons have been shown to underlie EEG effects that occur in humans and animals during loss of recall.[31]

Brain chemistry affects human consciousness. Sleeping drugs such as midazolam (Dormicum) can bring the brain from the awake condition (conscious) to the sleep (unconscious) condition. Wake-up drugs such as flumazenil reverse this process. Many other drugs (such as alcohol, nicotine, Tetrahydrocannabinol (THC), heroin, cocaine, LSD, MDMA, caffeine), have a consciousness-changing effect.[citation needed]

Neurophysiological studies in awake, behaving monkeys point to advanced cortical areas in prefrontal cortex and temporal lobes as carriers of neuronal correlates of consciousness. Christof Koch and Francis Crick argue that neuronal mechanisms of consciousness are intricately related to prefrontal cortex — cortical areas involved in higher cognitive function, affect, behavioral control, and planning. Rodolfo Llinas proposes that consciousness results from recurrent thalamo-cortical resonance where the specific thalamocortical systems (content) and the non-specific (centromedial thalamus) thalamocortical systems (context) interact in the gamma band frequency via time coincidence. According to this view the "I" represents a global predictive function required for intentionality.[32][33] Experimental work of Steven Wise, Mikhail Lebedev and their colleagues supports this view. They demonstrated that activity of prefrontal cortex neurons reflects illusory perceptions of movements of visual stimuli. Nikos Logothetis and colleagues made similar observations on visually responsive neurons in the temporal lobe. These neurons reflect the visual perception in the situation when conflicting visual images are presented to different eyes (i.e., bistable percepts during binocular rivalry). The studies of blindsight — vision without awareness after lesions to parts of the visual system such as the primary visual cortex — performed by Lawrence Weiskrantz and David P. Carey provided important insights on how conscious perception arises in the brain.[citation needed]

The Neuroscience of free will also seems to provide relevant insights to the understanding of consciousness.


Experimental research on consciousness presents special difficulties, e.g., when establishing whether an observer is unaware of a critical stimulus. Several techniques exist for dissociating the conscious visibility of stimuli from indirect effects they might have on behavior.[34] For example, the experimental technique of Response Priming allows researchers to find conditions where the conscious visibility of a critical stimulus and the ability of that stimulus to affect a motor response develop in opposite directions, e.g., when motor effects of a stimulus become larger under conditions where its visibility is decreasing.[35]

In medicine, several neurological and brain imaging techniques, such as EEG and fMRI, have proven useful for physical measures of brain activity associated with consciousness. This is particularly true for EEG measures during anesthesia, which can provide an indication of anesthetic depth.

Operational tests

It has been argued that due to the nature of the problem of consciousness, empirical tests are intrinsically impossible. However, several tests have been developed that attempt an operational definition of consciousness and try to determine whether computers and non-human animals can demonstrate through behavior, by passing these tests, that they are conscious.

Though sometimes thought of as a test for consciousness, the Turing test (named after computer scientist Alan Turing, who proposed it) was originally presented as an operational replacement for the question "Can machines think?", which Turing regarded as too ambiguous to be meaningful. This test is commonly cited in discussion of artificial intelligence. The test is based on an "Imitation Game" in which a human experimenter converses, via computer keyboards, with two competitors, one human, the other a computer. Because all of the conversation is by keyboard, no cues such as voice, prosody, or appearance will be available to indicate which is human and which is the computer. If the human judge is unable to determine which of the conversants is the computer, the computer is said to have "passed" the test. The Turing test has generated a great deal of philosophical debate. For example, Daniel Dennett and Douglas Hofstadter argue that anything capable of passing the Turing test is necessarily conscious,[36] while David Chalmers, argues that a philosophical zombie could pass the test, yet fail to be conscious.[37]

With the Mirror test, devised by Gordon Gallup in the 1970s, one is interested in whether animals are able to recognize themselves in a mirror. The classic example of the test involves placing a spot of coloring on the skin or fur near the individual's forehead and seeing if they attempt to remove it or at least touch the spot, thus indicating that they recognize that the individual they are seeing in the mirror is themselves. Humans (older than 18 months) and other great apes, bottlenose dolphins, pigeons, elephants[38] and magpies[39] have all been observed to pass this test. The test is usually carried out with an identical 'spot' being placed elsewhere on the head with a non-visible material as a control, to assure the subject is not responding to the touch stimuli of the spot's presence.

Biological function and evolution

Regarding the primary function of conscious processing, a recurring idea in recent theories is that phenomenal states somehow integrate neural activities and information-processing that would otherwise be independent (see review in Baars, 2002). This has been called the integration consensus. However, it remained unspecified which kinds of information are integrated in a conscious manner and which kinds can be integrated without consciousness. Obviously not all kinds of information are capable of being disseminated consciously (e.g., neural activity related to vegetative functions, reflexes, unconscious motor programs, low-level perceptual analyses, etc.) and many kinds can be disseminated and combined with other kinds without consciousness, as in intersensory interactions such as the ventriloquism effect (cf., Morsella, 2005).

Even among writers who consider consciousness to be a definite thing, there is widespread dispute about which animals other than humans can be said to possess it.[40] Thus, any examination of the evolution of consciousness is faced with great difficulties. Nevertheless, some writers have argued that consciousness can be viewed from the standpoint of evolutionary biology as an adaptation in the sense of a trait that increases fitness.[41] In his paper "Evolution of consciousness," John Eccles argued that special anatomical and physical properties of the mammalian cerebral cortex gave rise to consciousness.[42] Bernard Baars proposed that once in place, this recursive circuitry may have provided a basis for the subsequent development of many of the functions that consciousness facilitates in higher organisms.[43]

Other philosophers, however, have suggested that consciousness would not be necessary for any functional advantage in evolutionary processes.[44] It is possible, they argue, for a functionally equivalent non-conscious organism to achieve the same survival advantages as a conscious organism. If evolutionary processes are blind to the difference between function F being performed by conscious organism O and non-conscious organism O*, it is unclear what advantage consciousness would provide.[45]


Since the dawn of Newtonian science with its vision of simple mechanical principles governing the entire universe, some philosophers have been tempted by the idea that even consciousness could be explained in purely physical terms. The first influential writer to propose such an idea explicitly was Julien Offray de La Mettrie, in his book Man a Machine (L'homme machine).[46]

The most influential modern physical theories of consciousness are based on psychology and neuroscience. Theories proposed by neuroscientists such as Gerald Edelman[47] and António Damásio,[48] and by philosophers such as Daniel Dennett,[49] seek to explain access consciousness and phenomenal consciousness in terms of neural events occurring within the brain. Many other neuroscientists, such as Christof Koch,[50] have explored the neural basis of consciousness without attempting to frame all-encompassing global theories. At the same time, computer scientists working in the field of Artificial Intelligence have pursued the goal of creating digital computer programs that can simulate or embody consciousness.

Some theorists—most of whom are physicists—have argued that classical physics is intrinsically incapable of explaining the holistic aspects of consciousness, but that quantum theory provides the missing ingredients. Several theorists have therefore proposed quantum mind (QM) theories of consciousness; the most notable theories falling into this category include the Holonomic brain theory of Karl H. Pribram and David Bohm, and the Orch-OR theory formulated by Stuart Hameroff and Roger Penrose. Some of these QM theories offer descriptions of phenomenal consciousness, as well as QM interpretations of access consciousness. None of the quantum mechanical theories has been confirmed by experiment, and many scientists and philosophers consider the arguments for an important role of quantum phenomena to be unconvincing.[51]

States of consciousness

The three most widely accepted states of consciousness are sleeping, dreaming and waking. In addition to the obvious subjective differences each state has a corresponding pattern of physiological functioning in terms of EEG, biochemical markers, and (in dreaming) rapid eye movements.[52][53] There has been some research into physiological changes in yogis and people who practise various techniques of meditation. Recent research with brain waves during meditation has shown a distinct difference between those of just relaxation and the former.[54]

It is contested, however, whether there is enough evidence to count these as physiologically distinct states of consciousness.[55][56]


The merkwelt (German; English: "way of viewing the world", "peculiar individual consciousness") is a concept in robotics, psychology and biology that describes a creature or android's capacity to view things, manipulate information and synthesize to make meaning out of the universe.[citation needed] In biology, a shark's merkwelt for instance is dominated by smell due to its enlarged olfactory lobes whilst a bat's is dominated by its hearing, especially at ultrasonic frequencies. This term was particularly developed by the German biologist Jakob von Uexküll who framed it as part of his theory of umwelt. This basically stated that any living 'observer' of the broader environment or umwelt through their particular werkwelt or 'mechanical viewing' (that is to say, the organs through which they view the world- their eyes, ears, mouth etc. in humans and electrical sensors in sharks for instance) could have a merkwelt or 'perceptual universe'.[57]

Medical aspects

In medicine, consciousness is measured by neuropsychological assessment.[58] It is of concern to patients, physicians, and ethicists as well as biological scientists and biomedical engineers. Patients may suffer from disorders of consciousness and seek medical treatment. Physicians may perform medical interventions of consciousness such as instructing the patient to sleep, administering general anesthesia, or inducing medical coma. Bioethicists and neuroethicists may be concerned with the ethical implications of consciousness in medical cases of patients such as Karen Ann Quinlan[59] and Terri Schiavo.[60] Furthermore, biological scientists study patients with these disorders while biomedical engineers develop neuroprosthetics for them.


There are two commonly used methods for assessing the level of consciousness of a patient, a simple procedure that requires minimal training, and a more complex procedure that requires substantial expertise.

The simple procedure begins by asking whether the patient is able to move and react to physical stimuli. If so, the next question is whether the patient can respond in a meaningful way to questions and commands. If so, the patient is asked for name, current location, and current day and time. A patient who can answer all of these questions correctly is said to be "oriented times three" (sometimes denoted "Ox3" on a medical chart), and is usually considered fully conscious. A patient who can additionally describe the current situation may be referred to as "oriented times four".

The more complex procedure is known as a neurological examination, and is usually carried out by a neurologist in a hospital setting. A formal neurological examination runs through a precisely defined series of tests, beginning with tests for basic sensorimotor reflexes, and culminating with tests for sophisticated use of language. The outcome may be summarized using the Glasgow Coma Scale, which yields a number in the range 3—15, with a score of 3 indicating brain death (the lowest level of consciousness), and 15 indicating full consciousness. The Glasgow Coma Scale has three subscales, measuring the best motor response (ranging from "no motor response" to "obeys commands"), the best eye response (ranging from "no eye opening" to "eyes opening spontaneously") and the best verbal response (ranging from "no verbal response" to "fully oriented"). There is also a simpler pediatric version of the scale, for children too young to be able to use language.

Disorders of consciousness

Medical conditions that inhibit consciousness are considered disorders of consciousness.[61] This category generally includes minimally conscious state and persistent vegetative state, but sometimes also includes the less severe locked-in syndrome and more severe chronic coma.[61][62] Differential diagnosis of these disorders is an active area of biomedical research.[63][64][65] Finally, brain death results in an irreversible disruption of consciousness.[61] While other conditions may cause a moderate deterioration (e.g., dementia and delirium) or transient interruption (e.g., grand mal and petit mal seizures) of consciousness, they are not included in this category.

Disorder Description
Locked-in syndrome The patient has awareness, sleep-wake cycles, and meaningful behavior (viz., eye-movement), but is isolated due to quadriplegia and pseudobulbar palsy.
Minimally conscious state The patient has intermittent periods of awareness and wakefulness and displays some meaningful behavior.
Persistent vegetative state The patient has sleep-wake cycles, but lacks awareness and only displays reflexive and non-purposeful behavior.
Chronic coma The patient lacks awareness and sleep-wake cycles and only displays reflexive behavior.
Brain death The patient lacks awareness, sleep-wake cycles, and behavior.

Spiritual approaches


According to Vedanta, awareness is not a product of physical processes and can be considered under four aspects. The first is waking consciousness (jagaritasthana), the identification with “I” or “me” in relationship with phenomenal experiences with external objects. The second aspect is dream consciousness (svapna-sthana), which embodies the same subject/object duality as the waking state. The third aspect of consciousness is deep sleep (susupti), which is non-dual as a result of holding in abeyance all feelings, thoughts, and sensations. The final aspect is the consciousness that underlies and transcends the first three aspects (turiya) also referred to as a trans-cognitive state (anubhava) or a state of self-realization or freedom from body-mind identification (moksha).[66] The state of turiya has been correlated with physiological and bio chemical changes in the body.[67] Gaudiya Vedanta recognizes a fifth aspect of consciousness in which God becomes subordinate to bhakti.[68]


In Buddhism, consciousness (viññāṇa) is included in the five classically defined experiential "aggregates". The aggregates are seen as empty of self-nature; that is, they arise dependent on causes and conditions. The cause for consciousness arising (viññāa) is the arising of another aggregate (physical or mental); and, consciousness arising in turn gives rise to one or more of the mental (nāma) aggregates. The causation chain identified in the aggregate (khandha) model overlaps the conditioning chain in Dependent Origination (paticcasamuppāda) model.[69] Consciousness is the third link, between mind body mental formations and name & form in the traditional Twelve Causes (nidāna) of Dependent Origination.[70] The six classes of consciousness are: eye-consciousness, ear-consciousness, nose-consciousness, tongue-consciousness, body-consciousness, intellect-consciousness.[71] The following aspects are traditionally highlighted within Dependent Origination:

  • consciousness is conditioned by mental fabrications (saṅkhāra);
  • consciousness and the mind-body (nāmarūpa) are interdependent; and,
  • consciousness acts as a "life force" by which there is a continuity across rebirths.

See also


  1. van Gulick, 2004 [van Gulick, R. (2004). Consciousness. In Stanford Encyclopedia of Philosophy. (version Aug 16, 2004) http://plato.stanford.edu/entries/consciousness/]
  2. 2.0 2.1 Farthing, 1992 [Farthing, G. W. (1992). The Psychology of Consciousness. Upper Saddle River, NJ: Prentice Hall.]
  3. Searle, 2005, In Honderich, 2005
  4. Schneider and Velmans, 2007, pp.1-6 In Velmans & Schneider, 2007 [Velmans, M., & Schneider, S. (Eds.) (2007). The Blackwell Companion to Consciousness. Malden, MA: Blackwell.]; For a similar comment see also Güzeldere, 1995 In Block, Flanagan & Güzeldere, 1997, pp.1-67 [Block, N., Flanagan, O., & Güzeldere, G. (1997). The Nature of Consciousness: Philosophical debates Cambridge, MA: MIT.]
  5. James, W. 1910 In Block, Flanagan & Güzeldere [Block, N., Flanagan, O., & Güzeldere, G. (1997). The Nature of Consciousness: Philosophical debates Cambridge, MA: MIT.]
  6. cf. Searle, 2005 In Honderich, 2005, s.v. consciousness
  7. Samuel Butler first raised the possibility of mechanical consciousness in an article signed with the nom de plume Cellarius and headed "Darwin among the Machines", which appeared in the Christchurch, New Zealand, newspaper The Press on June 13, 1863: retrieved from PREFACE TO THE REVISED EDITION, Project Gutenberg eBook Erewhon, by Samuel Butler. Release Date: March 20, 2005.
  8. Stuart Shieber (ed): The Turing test : verbal behavior as the hallmark of intelligence, Cambridge, Mass, MIT Press, 2004, ISBN 978-0-262-69293-9
  9. Steven Marcus: Neuroethics: mapping the field. Dana Press, New York 2002. ISBN 978-0-9723830-0-4.
  10. Güzeldere, 1995 In Block, Flanagan & Güzeldere, 1997, pp.1-67 [Block, N., Flanagan, O., & Güzeldere, G. (1997). The Nature of Consciousness: Philosophical debates Cambridge, MA: MIT.]
  11. Late recovery from the minimally conscious state: ethical and policy implications. Fins JJ, Schiff ND, Foley KM. Neurology. 2007 January 23;68(4):304-7. Abstract at Pubmed, retrieved 27 February 2007
  12. Locke, John. "An Essay Concerning Human Understanding (Chapter XXVII)". Australia: University of Adelaide. http://ebooks.adelaide.edu.au/l/locke/john/l81u/B2.27.html. Retrieved August 20, 2010. 
  13. "Science & Technology: consciousness". Encyclopedia Britannica. http://www.britannica.com/EBchecked/topic/133274/consciousness. Retrieved August 20, 2010. 
  14. The Classic Latin Dictionary, Follett Publishing Company, 1957
  15. James Ussher, Charles Richard Elrington (1613). The whole works, Volume 2. Hodges and Smith. p. 417. 
  16. Hastings, James; Selbie (2003). Encyclopedia of Religion and Ethics Part 7. Kessinger Publishing. p. 41. ISBN 0766136779.  note: "In the sense of 'consciousness,' consientia is rare, but it is exceedingly common in most writers after Cicero with the meaning 'conscience'."
  17. Melenaar, G.. Mnemosyne, Fourth Series. 22. Brill. pp. 170–180.  note: reference only that Cicero had been using the word.[1]
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  69. This overlap is particularly pronounced in the Mahanidana Sutta (DN 15) where consciousness (viññāa) is a condition of name-and-body (nāmarūpa) and vice-versa (see, e.g., Thanissaro, 1997a).
  70. Not all canonical texts identify twelve causes in Dependent Origination's causal chain. For instance, the Mahanidana Sutta (DN 15) (Thanissaro, 1997a) identifies only nine causes (omitting the six sense bases, formations and ignorance) and the initial text of the Nalakalapiyo Sutta (SN 12.67) (Thanissaro, 2000) twice identifies ten causes (omitting formations and ignorance) although its final enumeration includes the twelve traditional factors.
  71. For instance, similar to the sensory-specific description of consciousness found in discussing "the All" (above), the "Analysis of Dependent Origination Discourse" (Paticcasamuppada-vibhanga Sutta, SN 12.2) describes viññāa ("consciousness") in the following manner:
    "And what is consciousness? These six are classes of consciousness: eye-consciousness, ear-consciousness, nose-consciousness, tongue-consciousness, body-consciousness, intellect-consciousness. This is called consciousness." (Thanissaro, 1997b)

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