The state of organisms that enables phenomenal experience and VOLUNTARY behaviour. Commonly, both the graded states of being conscious (as opposed to unconscious) and the content of a conscious experience are called consciousness. Consciousness is lost in COMA, ANAESTHESIA, and dreamless SLEEP, reduced in drowsiness and SOMNOLENCE, and fully present in attentive alertness and hyper-excitation. Loss and disturbance of consciousness can result from multiple causes including lack of oxygen, epileptic seizures, brainstem lesions, and substances like alcohol, barbiturates, ether, and ketamine. Clinically, the degree of disturbance is assessed with the GLASGOW COMA SCALE on the basis of eye opening, motor function, and verbal utterances. The states of consciousness are independent of its particular content: what one is conscious of is irrelevant as long as one is conscious of something. It is therefore not surprising that the degree to which an organism is conscious appears to be mediated by an unspecific neuronal system which involves the brainstem and DIENCEPHALON. In particular, the intralaminar nuclei of the THALAMUS have been suggested to be importantly involved, and were found to have been disproportionately affected in the coma case of Karen Ann Quinlan. This central and phylogenetically old system is present in other animals, as are SLEEP-WAKE CYCLES and REM SLEEP (rapid eye movement sleep) and non-REM phases of sleep which require SEROTONIN and NORADRENALINE release.
In contrast, what an organism can be conscious of once it is in a conscious state depends primarily on its sensory equipment, which in turn depends on the ecological niche in which it evolved. Sensory systems differ widely even among vertebrates, ranging from vision and audition tuned to different parts of the electromagnetic and frequency spectra, to sonar systems and sideline organs. Note that sensory processing per se need not be consciously represented and is thus not sufficient to attribute consciousness to an organism. Neuroendocrine and reflexive responses can be elicited from comatose and sleeping subjects and do not require the organism to be conscious. In addition, subjects can be in a conscious state and nevertheless not have a conscious perception in response to sensory stimulation. In normal subjects, paradigms applying subliminal and masked stimulation, or BINAURAL and BINOCULAR RIVALRY, have revealed that unperceived information can exert noticeable influences on the conscious voluntary response. Similarly, in patients with lesions to the central parts of the sensory systems who have lost the conscious representation of stimuli of a particular sensory modality, effects of the unperceived stimuli upon non-reflexive responses have been extensively demonstrated. Examples of such implicit processes come from visual BLINDSIGHT, auditory (deaf hearing), somatosensory (unfeeling touch) and memory domains. Their discovery and description may open empirical avenues to two important questions. The first regards the purpose or function of conscious representations, and can be tackled by studying what is and what is not possible based on sensory information that is demonstrably processed but not consciously represented.
The second regards the neuronal substrate mediating conscious representations. What structures are needed? The loss of which structures prevents information from becoming conscious? Are the structures, pathways, neurons, neurotransmitters and neuromodulators that mediate only unconscious sensory processing different from those that mediate conscious perception? Is neocortical participation required for conscious perception? Are all functional sub-divisions of NEOCORTEX equally capable of mediating conscious content?
Neuropsychological data show that different aspects of conscious perception even within a single modality can dissociate following a LESION to different parts of the system. The degree of dissociability is astonishing: MEMORY and LANGUAGE can be lost selectively for narrow groups of objects, object IMAGERY can persist in the absence of OBJECT RECOGNITION, and ALEXIA can occur without AGRAPHIA. The selective loss of pieces of the representation caused by circumscribed lesions demonstrates PARALLEL PROCESSING, but conscious perception also seems to be hierarchically structured. The phenomenal representation of sensory information, for instance as colour or sound, appears prior to the conscious identification of the object having the colour or emitting the sound. In contrast to the unity of experience that many philosophers and psychologists have posited, these findings demonstrate a modularity of the mind. HEMISPHERIC SPECIALIZATION, notably observed in SPLIT BRAIN patients whose CORPUS CALLOSUM has been cut to control epileptic seizures, is a notable feature of this modular organization. LANGUAGE, predominantly represented in one HEMISPHERE, helps in assessing consciousness in both meanings of the term, that of being conscious, and that of being conscious of. Being our major means to exchange and access abstract ideas, it plays a role for the social components of consciousness, and contributes to its contents. Both quality and quantity of content are important for the normal function, disturbances resulting from traumatic life events, from too little input (as in SENSORY DEPRIVATION), and from an overextension of its limited capacity (for example, in certain psychiatric disorders or following sleep deprivation).
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PETRA STOERIG
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