Blindsight is a phenomenon in which people who are perceptually blind in a certain area of their visual field demonstrate some response to visual stimuli.[1][2] In Type 1 blindsight subjects have no awareness whatsoever of any stimuli, but yet are able to predict, at levels significantly above chance, aspects of a visual stimulus, such as location, or type of movement, often in a forced-response or guessing situation. Type 2 blindsight is when subjects have some awareness of, for example, movement within the blind area, but no visual percept. This may be caused by, for example, the person being aware of their eyes' tracking motion which will function normally. Blindsight is caused by injury to the part of the brain responsible for vision (see occipital lobe). Evidence for it can be indirectly observed in children as young as two months, although it is difficult to determine the type in a person who is not old enough to answer questions.[3]
Contents |
Visual processing in the brain goes through a series of stages. Destruction of the first visual cortical area, primary visual cortex (also known as V1 or striate cortex) leads to blindness in the part of the visual field that corresponds to the damaged cortical representation. The area of blindness - known as a scotoma - is in the visual field opposite the damaged hemisphere and can vary from a small area up to the entire hemifield.
Although individuals with damage to V1 are not consciously aware of stimuli presented in their blind field, Lawrence Weiskrantz and colleagues showed in the early 1970s that if forced to guess about whether a stimulus is present in their blind field, some observers do better than chance.[4] This ability to detect stimuli that the observer is not conscious of can extend to discrimination of the type of stimulus (for example, whether an 'X' or 'O' has been presented in the blind field). This general phenomenon has been dubbed "blindsight".
It is unsurprising from a neurological viewpoint that damage to V1 leads to reports of blindness. Visual processing occurs in the brain in a hierarchical series of stages (with much crosstalk and feedback between areas). The route from the retina through V1 is not the only visual pathway into the cortex, though it is by far the largest; it is commonly thought that the residual performance of people exhibiting blindsight is due to preserved pathways into the extrastriate cortex that bypass V1. What is surprising is that activity in these extrastriate areas is apparently insufficient to support visual awareness in the absence of V1.
Blindsight may be thought of as a converse of the form of anosognosia known as Anton–Babinski syndrome, in which there is full cortical blindness along with the confabulation of visual experience.
Medical fiction author Robin Cook writes extensively on this condition.
It helps reveal the existence and importance of an unconscious force as an aspect for human behavior. [5] It can also help explain how some experiences classified as "magical" or "transcendent", or even for characteristics of individuals that they themselves can't identify. A dissociation between unconscious and conscious processing is an important aspect of learning, since the two utilise experience in various ways.[6]
Colin McGinn sees in the phenomenon of blindsight one reason for his thesis of a natural depth of consciousness.[7] Robert Nozick also joins McGinn in supporting this thesis.[8]