"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 [20] . It is likely
that different components of consciousness can be teased apart with
anesthetics, sedatives and hypnotics. These drugs appear to
differentially act 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
(MacIver et al 1996; see also: http://www.stanford.edu/group/maciverlab/research.html).

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. At present, the 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.[citation needed]

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 alcohol, nicotine,
Tetrahydrocannabinol (THC), 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. This
procedure was first performed by Roger Sperry in the 1960s. Tests of
these patients have shown that, after the link is completely severed,
the hemispheres are no longer able to communicate, leading to certain
problems that usually arise only in test conditions. For example,
while the left side of the brain can verbally describe what is going
on in the right visual field, the right hemisphere is essentially
mute, instead relying on its spatial abilities to interact with the
world on the left visual field. Some say that 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.[citation needed]

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. This evidence
suggests that a functioning thalamus is necessary, but not sufficient,
for human consciousness.

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 argued that neuronal mechanisms of consciousness are
intricately related to prefrontal cortex — the most advanced cortical
area. Rodolfo Llinas proposes that consciousness results from
recurrent thalamo-cortical resonance where the specific (dorsal
thalamus) thalamocortical system (content) and the non-specific
(centromedial thalamus) thalamocortical system (context) interact at
gamma band frequency via time coincidence. According to this view the
"I" represents a global predictive function required for
intentionality.[21][22] 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. In recent years the
theory of two visual streams, vision for perception versus vision for
action was developed by Melvyn Goodale, David Milner and others.
According to this theory, visual perception arises as the result of
processing of visual information by the ventral stream areas (located
mostly in the temporal lobe), whereas the dorsal stream areas (located
mostly in the parietal lobe) process visual information unconsciously.
For example, quick catching of the ball would engage mostly the dorsal
stream areas, and viewing a painting would be handled by the ventral
stream. Overall, these studies show that conscious versus unconscious
behaviors can be linked to specific brain areas and patterns of
neuronal activation.[citation needed]. However, neuroscience only
focuses on the neural correlates of consciousness. The hard problem of
consciousness is to explain how all these flows and electrochemical
processes in the brain give rise to the inner experience of subjective
awareness.

One of the promising approaches in modern Neuroscience is Operational
Architectonicstheory of brain-mind functioning developed by Andrew and
Alexander Fingelkurts. This theory states that whenever any pattern of
phenomenality (including reflective thought) is instantiated, there is
neuro-physiological pattern (revealed directly by EEG) of appropriate
kind that corresponds to it.[23][24] These neuron-physiological EEG
patterns (expressed as the virtual operational modules) are brought to
existence by joint operations of many functional and transient
neuronal assemblies in the brain.[25] The activity of neuronal
assemblies per se is 'hidden' in the complex nonstationary structure
of EEG field.[26] Therefore, a proper EEG analysis is needed that
would be able to reveal the EEG architecture which reflects or
instantiates the kind of phenomenal world (considering that there
should be the ‘well-defined’ and ‘well-detected’ EEG phenomena) which
humans subjectively experience. Currently available EEG methods can
reveal the EEG architecture which is amazingly similar to the
architecture of a phenomenal world of consciousness (see review on EEG
and Operational Architectonics)."

http://en.wikipedia.org/wiki/Consciousness#Cognitive_neuroscience