The Three Classes of Consciousness Definition:

 The first is the definition of consciousness as a state. Most mammals, and many non-mammalian species as well, have this aspect of conscious capacity. All mammals alternate between states that can be labeled as asleep, awake, and alert. Several species of mammals even seem to dream as we do and show rapid eye movements and brain waves that resemble our own during certain phases of asleep. Most mammals also show daytime variation in arousal; i.e., they alternate between what we call active, vigilant, and wide-awake states and those that are passive, unfocused, and marked by a reduced level of activity.

A second class of definition takes an architectural approach, whereby consciousness is considered as the central processor located at the top of the control hierarchy. Its physical counterpart is distributed across several subsystems of anatomical structures. These subsystems are identified to be the various thalamic nuclei located in the thalamus. They are the mediator between the unconscious atuonomic functions in the brain stem and conscious awareness - the higher mental activities in the forebrain. This conceptual organization brings cognition, emotion, and action under a unified command. Consciousness would correspond to the active state of this command center in the thalamus. For instance, when suddenly you feel the need for a drink. This "need" is the thalamus, on behalf of its autonomic subsystem, breaking through into your conscious awareness and modifying your behavior to meet your body's basic needs.

Here's how it works:

Inputs arrive in the thalamus as nerve signals. They may be from the senses, especially sight, taste, and touch; from the limbic system or recticular formation in the brain itself (about basic need and drives); and from internal organs such as the heart and intestines. It is also influenced by hormones circulating in the blood, and by concentrations of glucose, sodium, and other substances in the blood passing through it and in the cerebrospinal fluid just above it. Inside the thalamus are several pairs of relatively discrete nuclei. Some tasks of the thalamus are assigned to a specific pair; others seem to be spread through several areas which communicate by nerve fibers so that they can work together as a single "center" for one main role.

The thalamus sends outputs as nerve signals to the motor (muscle controlling) parts of the midbrain, the limbic system, and the various autonomic centers in the brain stem that control processes such as heartbeat, blood pressure, and urine production, as well as the glands producing saliva, sweat, and digestive juices. It also produces hormones - such as the antidiuretic hormone (ADH) - which are passed to the pituitary.

The late Francis Crick (1916-2004) believed that the command center is in the claustrum instead of the thalamus. The claustrum is a thin sheet of grey matter that resides parallel to and below part of the cortex. It is present in all mammals, but it has been little studied and its function is not known. What is known, however, is that there are two-way connections between the claustrum and most, if not all, parts of the cortex as well as subcortical structures involved in emotion. It is likened to the conductor of an orchestra, who is responsible for binding the performances by individual musicians into an integrated whole that can be much more than the sum of its parts. The neuroanatomical connections of the claustrum, then, just match with the "conductor" required to bind together the various disparate components of the conscious experience represented in many different brain regions.

The third definition of consciousness takes a human-centered view of cognition and has more to do with enlightenment, or illumination, than with mere attention. This is the representational approach, in which consciousness is made dependent upon our human capacity for symbolization. Language is one example of representing something in symbolic form, which expands the power of storing knowledge and information enormously both in space and time (in the forms of books, CD-ROMs, and libraries, ...). This definition relates consciousness exclusively to mental process in the cortex of a human brain.

Recent research indicates that consciousness is involved only in activities stemming from the associative regions of the cortex. These regions are found in the neocortex, which consists of four lobes: the occipital, patietal, temporal and frontal. The associative cortex is involved with the conscious perception and identity of one's own body; in the planning of movement, spatial perception, orientation and imagination; and in spatial alertness. The development of consciousness seems to be largely reliant on the numerous nerve cells in the cortex being linked to each others. The cortex's high number of connections vastly exceeds the number of points of entry and exit. This arrangement means that the cortex communicates with itsef more than with the sensory organs and motor apparatus.

Recently in 2007, it is suggested that all the above are "red herrings". Consciousness does not depend on language as babies, and many animals are not insensate robots. Nor can consciousness be equated with self-awareness. At times we have all lost ourselves in music, exercise or sensual pleasure, but that is different from being knocked out cold. Even the central control architecture is an illusion. Consciousness turns out to consist of a maelstrom of events distributed across the brain. These events compete for attention, and as one process out-shouts the others, the brain rationalizes the outcome after the fact and concocts the impression that a single self was in charge all along.

Most neuroscientists now believe that all aspects of mind, including its most puzzling attribute -- consciousness or awareness -- are likely to be explainable in a more materialistic way as the behavior of large sets of interacting neurons.
Modern explanation of consciousness is based on "neural correlate", which assumes:

The subjective content associated with a conscious sensation does exist and has its physical basis in the brain.

All the different aspects of consciousness (smelling, pain, visual awareness, self-consciousness, and so on) employ a basic mechanism.

Consciousness is a property of the human brain, a highly evolved system. It therefore must have a useful function to perform, e.g., to produce the best current interpretation of the environment and to make it available to the parts of the brain which contemplate, plan and execute voluntary motor outputs (including language).

At least some animal species posses some aspects of consciousness.

It is believed that consciousness is not a thing but a process. The idea of "neural correlates of consciousness" has appeared only in the past few years. It is known that consciousness is always associated with neural activity in a complex brain, and that altering that activity, whether by changes in sensory input, injury, disease, drugs, direct electrical stimulation or neurosurgery, can alter the contents of consciousness. So a reasonable hypothesis is that individual conscious states exist only in association with particular patterns of neural activity. It is these patterns that are referred to as the neural correlates of consciousness (NCC). An example is on the NCC of visual consciousness in monkeys. It has now been determined that neurons in the inferior temporal cortex are responsible for the monkeys to see horizontal or vertical lines. See Higher Functions in Nervous System for additional information.