Cognitive neuroscience approaches:

Modern investigations into and discoveries about consciousness are based on psychological statistical studies and case studies of consciousness states and the deficits caused by lesions, stroke, injury, or surgery that disrupt the normal functioning of human senses and cognition. These discoveries suggest that the mind is a complex structure derived from various localized functions that are bound together with a unitary awareness.

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.

Loss of consciousness also occurs in other conditions, such as general (tonic-clonic) epileptic seizures, in general anaesthesia, maybe even in deep (slow wave) sleep. The currently best supported hypotheses about such cases of loss of consciousness (or loss of time resolution) focus on the need for 1) a widespread cortical network, including particularly the frontal, parietal and temporal cortices, and 2) cooperation between the deep layers of the brain, especially the thalamus, and the upper layers; the cortex. Such hypotheses go under the common term "globalist theories" of consciousness, due to the claim for a widespread, global network necessary for consciousness to interact with non-mental reality in the first place.

Brain chemistry affects human consciousness. Sleeping drugs (such as Midazolam = Dormicum) can bring the brain from the awake condition (conscious) to the sleep (unconscious). Wake-up drugs such as Anexate reverse this process. Many other drugs (such as heroin, cocaine, LSD, MDMA) have a consciousness-changing effect.

There is a neural link between the left and right hemispheres of the brain, known as the corpus callosum. This link is sometimes surgically severed to control severe seizures in epilepsy patients. Tests of these patients have shown that after the link is completely severed, each hemisphere possesses its own sense of self and each has a separate awareness from the other. It is as if two separate minds now share the same skull, but both still represent themselves as a single "I" to the outside world.

The bilateral removal of the Centromedian nucleus (part of the Intra-laminar nucleus of the Thalamus) appears to abolish consciousness, causing coma, PVS, severe mutism and other features that mimic brain death. The centromedian nucleus is also one of the principal sites of action of general anaesthetics and anti-psychotic drugs.

Philosophical approaches:

Philosophers distinguish between phenomenal consciousness and access or psychological consciousness. Some suggest that consciousness resists or even defies definition. There are many philosophical stances on consciousness, sometimes known as 'isms', including: behaviorism, cognitivism, dualism, idealism, functionalism, phenomenalism, physicalism, pseudonomenalism, emergentism, and mysticism.

Phenomenal and access consciousness:

Philosophers call our current experience phenomenal consciousness. Phenomenal consciousness is simply experience, it is moving, coloured forms, sounds, sensations, emotions and feelings with our bodies and responses at the centre. The Hard problem of consciousness is how to explain a state of phenomenal consciousness in terms of its neurological basis (Chalmers 1996). Some philosophers, such as Descartes in his famous phrase "cogito, ergo sum", believe that phenomenal consciousness is incorrigible, meaning that it cannot be doubted.

Access consciousness means something like awareness, or that a mind is directed at something. (That sounds more like a definition of that philosophical term "intentionality" often referred to with the layman's term "aboutness".) So when we perceive, we are conscious of what we perceive; when we introspect, we are conscious of our thoughts; when we remember, we are conscious of something that happened in the past, or of some piece of information that we learned; and so on. Naive and Direct Realists believe that access consciousness is all that needs to be known about consciousness because they regard phenomenal consciousness as the world itself.

Events that occur in the mind or brain that are not within phenomenal consciousness are known as unconscious events.

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