The primate visual system is the best-characterized sensory system and has therefore been used as a model in which to study the neural correlates of visual consciousness. Electrophysiological studies in monkeys and functional neuroimaging studies in humans can be used to address this issue. To identify the neural correlates of conscious experience (as opposed to simply being conscious), it is necessary to dissociate the neural activity that correlates with a single conscious experience from activity that reflects unconscious perception or action associated with that experience.
Activity in primary visual cortex (V1) is necessary for conscious perception of a visual stimulus. However, some of the information represented in V1 (such as which eye a stimulus is presented to) is not available to consciousness, and activity in V1 does not always correlate with conscious experience. The current evidence supports the idea that activity in V1 is necessary but not sufficient for conscious perception.
Activity in areas of extrastriate visual cortex correlates more closely with visual perception, and damage to these areas can selectively impair the ability to perceive particular features of stimuli. But there is also some evidence that the correlation between activity in extrastriate cortex and conscious experience is not perfect.
It is possible that the timing or synchronization of neural activity, rather than simply the overall level of spiking, might correlate with or mediate awareness. Although there is some evidence to support this theory, it has not been possible to show in primates that disrupting synchrony of firing causes any perceptual impairment. The evidence that addresses these ideas is preliminary.
Recent neuroimaging studies have indicated that activity in areas of parietal and prefrontal cortex might also be associated with visual awareness. The authors suggest that activity in extrastriate visual cortex might require an additional contribution from these areas to mediate awareness. Neuropsychological evidence from patients with damage to the parietal or prefrontal cortices, in whom disturbances of visual attention and visual awareness can occur, supports this theory, although in these patients awareness is disturbed but not eliminated.
A better understanding of the neural correlates of consciousness in specialized areas of extrastriate visual cortex will help us to understand awareness. Another important area for study will be the interactions between ventral and dorsal areas.
The directness and vivid quality of conscious experience belies the complexity of the underlying neural mechanisms, which remain incompletely understood. Recent work has focused on identifying the brain structures and patterns of neural activity within the primate visual system that are correlated with the content of visual consciousness. Functional neuroimaging in humans and electrophysiology in awake mokeys indicate that there are important differences between striate and extrastriate visual cortex in how well neural activity correlates with consciousness. Moreover, recent neuroimaging studies indicate that, in addition to these ventral areas of visual cortex, dorsal prefrontal and parietal areas might contribute to conscious visual experience.
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Encyclopedia of Life Sciences
MIT Encyclopedia of Cognitive Sciences
- ASCENDING RETICULAR ACTIVATING SYSTEM
A loosely organized core of neurons that extends from the midbrain to the thalamus. These neurons project widely to the cerebral hemispheres and are thought to be involved in maintaining the alert state.
A neurological syndrome (often involving damage to the right parietal cortex) in which patients show a marked difficulty in detecting or responding to information in the contralesional field.
The ability of a person with a lesion in the primary visual cortex to reach towards or guess at the orientation of objects projected on the part of the visual field that corresponds to this lesion, even though they report that they can see nothing in that part of their visual field.
Small, abrupt eye movements that occur constantly during normal vision.
- TOPOGRAPHICAL AGNOSIA
A condition in which patients become disorientated because they can no longer recognize their environment.
- VISUAL EXTINCTION
This is often associated with damage to the parietal cortex. The patient can see a stimulus presented alone in the contralateral visual field, but cannot see it if it is presented at the same time as a stimulus in the ipsilateral visual field.
The inability to perceive moving stimuli.
The inability to perceive colours.
Classically, a type of visual defect in which the patient cannot identify a familiar face, even though they know that a face is a face and can point out the features.
- EXTRASTRIATE CORTEX
A belt of visually responsive areas of cortex surrounding the primary visual cortex.
- FLASH SUPPRESSION
The perceptual suppression of a monocular stimulus on flashing a different stimulus to the opposite eye, while keeping the original stimulus on the other eye.
- ATTENTIONAL BLINK
The brief time after responding to an attended visual stimulus during which a subsequent visual stimulus cannot be seen or reported accurately.
- N400 EVOKED POTENTIAL
A characteristic waveform that appears 400 ms after presentation of a target word and is considered to represent an index of language processing.
- BINDING PROBLEM
The problem of binding together in a single unified percept the different features of an object, which are represented in different locations in the brain. For multiple objects, binding the correct features of each object to its perceptual representation is a particularly complex problem.
- STRABISMIC CATS
Strabismus is a condition in which the eyes are not straight or properly aligned. The misalignment reflects the failure of the eye muscles to work together. One eye might turn in (crossed eyes), turn out (wall eyes), turn up or turn down. Although some cats are congenitally strabismic, strabismus can also be achieved by cutting the tendon of one of the eye muscles.
- STEADY-STATE NEUROMAGNETIC RESPONSES
Changes in electrical activity in the brain produce tiny magnetic field perturbations that can be detected as neuromagnetic responses using an array of magnetometers placed on the scalp in a magnetoencephalographic scanner.
- CLASSICAL RECEPTIVE FIELD
The region of the visual field in which visual stimulation typically causes a response in a particular neuron.
- STEREO POP-OUT
Presenting each eye with two slightly different images that create a binocular disparity gives rise to a vivid impression of stereo depth. If the images are intricate patterns of dots (forming a shape) that are embedded in a similar background, then this perception of depth is not instantaneous — the figure 'pops out' after a short period of viewing.
- BALINT'S SYNDROME
A neurological disorder caused by bilateral damage to the parieto-occipital region of the brain and characterized by disorders of spatial perception.
An impairment in the ability to perceive and attend to all of the features of an object or a scene at the same time. Sufferers perceive the individual parts of a complex visual display better than they do the whole.
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Rees, G., Kreiman, G. & Koch, C. Neural correlates of consciousness in humans. Nat Rev Neurosci 3, 261–270 (2002). https://doi.org/10.1038/nrn783
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