Blindsight
From Psy3241
Contents |
What it is.
Blindsight is an extraordinary neuropsychological phenomenon that occurs after certain kinds of brain lesions, primarily in area V1, causing blindness. It is the ability to respond accurately (better than chance) to visual inputs given, without having the feeling or ability to see them. There are two types of Blindsight. The first type involves people that have no awareness of any objects projected into their blind region, but are able to correctly identify where the object is when pressed to do so. The second type of Blindsight involves people who have awareness of movement in their blind visual field, and are able to accurately ‘guess’ where the object is located.
How it happens
The primary visual cortex is used for conscious vision. Damage to area V1, where the primary visual cortex is located, causes blindness in the opposite visual field of the hemisphere in which the damage was done. For Blindsight to be understood, vision must be understood. Visual processing occurs when ganglion cells of the retina send axons through the optic nerve to the lateral geniculate nucleus. The lateral geniculate nucleus is located in the thalamus and is a primary processor of visual information. It receives the visual information and then projects it to the primary visual cortex, or V1. Information from V1 is then sent to the secondary visual cortex and onto the occipital areas of the ventral and dorsal streams. The dorsal stream projects to the parietal lobe and the ventral stream sends information to the temporal lobe. Damage to the dorsal stream(area’s V3 and V5) causes cerebral akinetopsia (V5) and Balint’s syndrome. Damage to the ventral stream (area’s V4 and V8) causes cerebral achromotopsia (V4) and visual agnosia (V8). Thus it is clear that when there is damage to V1, the visual information processing stream is interrupted and visual deficits result. It is hypothesized that Blindsight and normal vision are controlled by other sections of the brain as a backup to the primary visual cortex. The superior colliculus, for instance, has been proven to take over some of the functions of the primary visual cortex. The superior colliculus supplies blindsighted individuals with the intrinsic feeling of where the object is in their blind visual field. It is used as a reflex for orienting eye and head movements towards something that is seen or heard. The only brain region though that allows for complete sight, local orientation, spatial frequency, and color properties within small receptive fields, is the primary visual cortex.
