Cognitive neuroscience

Primary visual cortex and visual awareness

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Abstract

The primary visual cortex (V1) is probably the best characterized area of primate cortex, but whether this region contributes directly to conscious visual experience is controversial. Early neurophysiological and neuroimaging studies found that visual awareness was best correlated with neural activity in extrastriate visual areas, but recent studies have found similarly powerful effects in V1. Lesion and inactivation studies have provided further evidence that V1 might be necessary for conscious perception. Whereas hierarchical models propose that damage to V1 simply disrupts the flow of information to extrastriate areas that are crucial for awareness, interactive models propose that recurrent connections between V1 and higher areas form functional circuits that support awareness. Further investigation into V1 and its interactions with higher areas might uncover fundamental aspects of the neural basis of visual awareness.

Key Points

  • There are two main theories that pertain to the role of the primary visual cortex (V1) in visual awareness. Hierarchical models propose that although V1 provides necessary input, only high-level extrastriate areas that project to frontal-parietal attentional areas are directly involved in awareness. Interactive models propose that dynamic recurrent circuits between V1 and higher areas are necessary to maintain a visual representation in awareness. These models yield different predictions about whether awareness will be impaired by V1 disruption if extrastriate activity remains intact.

  • V1 damage severely impairs visual awareness, indicating that this region is necessary for normal conscious vision. Lesions to extrastriate areas lead to more specific visual deficits, whereas damage to parietal and/or superior-temporal areas can lead to gross visual neglect of contralateral space. Therefore, no single brain area is sufficient for visual awareness. Nonetheless, V1 seems to be the only single cortical area that is crucial for visual awareness.

  • Some subjects with V1 lesions can make accurate forced-choice visual discriminations in the absence of reported awareness. These implicit residual abilities (blindsight) presumably reflect the sustained activity that is found in many extrastriate areas, including motion-sensitive areas MT and V3A, and object-sensitive areas V4/V8 and the lateral occipital area. Similarly, motion phosphenes elicited by transcranial magnetic stimulation of area MT can be disrupted by subsequent stimulation to V1, indicating that extrastriate activity alone might be insufficient for awareness and that feedback projections from MT to V1 may be important for awareness of motion.

  • V1 activity is strongly associated with awareness under certain ambiguous perceptual conditions. During binocular rivalry, awareness spontaneously alternates between two competing monocular images. Human neuroimaging studies have revealed strong awareness-related modulations in V1 during rivalry. Likewise, neurophysiological and functional magnetic resonance imaging studies of visual detection tasks have found that V1 activity is greater for perceived than unperceived targets, and that the degree of response enhancement can predict detection performance.

  • However, not all studies have found a consistent relationship between V1 activity and awareness, including those of internally generated visual experiences (such as hallucinations, dreaming or imagery). In some studies, changes in perception are associated with increased extrastriate activity and concomitant decreases in V1 activity, indicating a more complex relationship.

  • Current evidence indicates that V1 activity is necessary for normal conscious perception and is closely associated with some forms of visual awareness. Further investigation of V1 and its interactions with higher areas might provide important insights into the neural basis of visual awareness.

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Figure 1: Connections between a subset of cortical visual areas (schematic diagram).
Figure 2: Extrastriate activations to objects in the absence of primary visual cortex (V1) and reported awareness.
Figure 3: Functional magnetic resonance imaging correlates of binocular rivalry in human primary visual cortex.
Figure 4: Multi-unit activity in primary visual cortex correlates with conscious detection of visual figures on a background.
Figure 5: Relationship between timing of primary visual cortex disruption and visual awareness.

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Acknowledgements

I would like to thank C. Gross, S. Kastner, T. Moore and A. Seiffert for helpful comments on this manuscript. This work was supported by the National Institutes of Health, James S. McDonnell Foundation and Pew Charitable Trusts.

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FURTHER INFORMATION

Encyclopedia of Life Sciences

brain imaging: localization of brain functions

brain imaging: observing ongoing neural activity

magnetic resonance imaging

MIT Encyclopedia of Cognitive Science

consciousness

magnetic resonance imaging

positron emission tomography

top–down processing in vision

Glossary

PRIMARY VISUAL CORTEX

The first cortical area to receive inputs from the eye via the geniculostriate pathway; also referred to as V1, area 17 and striate cortex.

EXTRASTRIATE CORTEX

A belt of visually responsive areas of cortex surrounding the primary visual cortex.

MT AND MST

Motion sensitive areas of extrastriate cortex.

PO AND PIP

The parieto-occipital (PO) and posterior intraparietal (PIP) visual areas lie in the dorsal stream and have weak reciprocal connections with V1. Their specific functions are not well understood.

FST

This visual area lies anterior to MT and MST in the floor of the superior temporal sulcus, and is also involved in motion perception but has not been extensively studied.

STP

The superior temporal polysensory area contains neurons that respond to visual, auditory and somatosensory stimuli, and responds strongly to visual motion.

TOE AND TE

These areas comprise the posterior and anterior portions of inferotemporal cortex (IT) respectively, and are involved in shape, object and face processing.

TH

This visual area lies in the parahippocampal gyrus, which has been implicated in scene perception and visual memory.

LIP

The lateral intraparietal area (LIP) is strongly implicated in visual-spatial attention and eye movement planning.

FRONTAL EYE FIELDS

(FEF). These areas are strongly implicated in visual–spatial attention and eye movement planning, and have strong connections with area LIP.

DORSAL STREAM

Visual brain areas that are involved in the localization of objects and are mostly found in the posterior/superior part of the brain.

VENTRAL STREAM

Visual brain areas that are involved in the identification of objects and are mostly found in the posterior/inferior part of the brain.

BACKWARD VISUAL MASKING

The reduced perception that occurs when a weak or brief stimulus is followed immediately by a stronger stimulus.

SYNAESTHESIA

An unusual 'mixing of the senses' in which a stimulus in one sensory modality (for example, a sound) elicits a percept in another modality (such as visual perception of a colour).

HEMIANOPIA

Loss of vision over half of the visual field, typically resulting from damage to the optic radiations that project to V1 or damage to V1 itself.

MOTION PHOSPHENES

Moving visual images that can be induced by stimulating parts of the visual system that are sensitive to motion.

ORTHODROMIC ACTIVATION

Activation of a target neuron by stimulation of an input neuron that synapses onto the target; action potentials are propagated in the normal direction along the input axon.

TRANSCRANIAL MAGNETIC STIMULATION

(TMS). A technique that is used to induce a transient interruption of normal activity in a relatively restricted area of the brain. It is based on the generation of a strong magnetic field near the area of interest, which, if changed rapidly enough, will induce an electric field that is sufficient to stimulate neurons.

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