Blindsight
Encyclopedia
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. 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.
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. 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). As V1 is the first cortical area in this hierarchy, any damage to V1 severely limits visual information passing from the retina
, via the LGN
and then V1, to higher cortical areas. However, 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.
sees in the phenomenon of blindsight one reason for his thesis of a natural depth of consciousness. Robert Nozick
also joins McGinn in supporting this thesis.
Blindness
Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...
in a certain area of their visual field demonstrate some response to visual stimuli. 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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
responsible for vision (see occipital lobe
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
). 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.
Technical details
Visual processingVisual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
in the brain goes through a series of stages. Destruction of the first visual cortical area, primary visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
(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
Scotoma
A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated visual acuity which is surrounded by a field of normal - or relatively well-preserved - vision.Every normal mammalian eye has a scotoma in its field of vision, usually...
- 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
Lawrence Weiskrantz
Lawrence Weiskrantz is a British psychologist, who discovered the phenomenon of blindsight, which is the voluntary visually evoked response to a stimulus presented within a scotoma.-Career:* Part-time Lecturer, Tufts University, 1952...
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. 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
Hierarchy
A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...
series of stages (with much crosstalk and feedback between areas). As V1 is the first cortical area in this hierarchy, any damage to V1 severely limits visual information passing from the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, via the LGN
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
and then V1, to higher cortical areas. However, 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
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
that bypass V1. What is surprising is that activity in these extrastriate areas is apparently insufficient to support visual awareness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
in the absence of V1.
Blindsight may be thought of as a converse of the form of anosognosia
Anosognosia
Anosognosia /æˌnɒsɒgˈnəʊsɪə/ is a condition in which a person who suffers disability seems unaware of the existence of his or her disability. Unlike denial, which is a defense mechanism, anosognosia is rooted in physiology...
known as Anton–Babinski syndrome, in which there is full cortical blindness
Cortical blindness
Cortical blindness is the total or partial loss of vision in a normal-appearing eye caused by damage to the visual area in the brain's occipital cortex. This damage is most often caused by loss of blood flow to the occipital cortex from either unilateral or bilateral posterior cerebral artery...
along with the confabulation
Confabulation
Confabulation is the process in which a memory is remembered falsely. Confabulations are indicative of a complicated and intricate process that can be led astray at any given point during encoding, storage, or recall of a memory. Two distinct types of confabulation are often distinguished...
of visual experience.
Medical fiction author Robin Cook
Robin Cook (novelist)
Dr. Robin Cook is an American physician and novelist who writes about medicine and topics affecting public health....
writes extensively on this condition.
Implications
It helps reveal the existence and importance of an unconscious force as an aspect for human behaviour. 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.Philosophical reception
Colin McGinnColin McGinn
Colin McGinn is a British philosopher currently working at the University of Miami. McGinn has also held major teaching positions at Oxford University and Rutgers University. He is best known for his work in the philosophy of mind, though he has written on topics across the breadth of modern...
sees in the phenomenon of blindsight one reason for his thesis of a natural depth of consciousness. Robert Nozick
Robert Nozick
Robert Nozick was an American political philosopher, most prominent in the 1970s and 1980s. He was a professor at Harvard University. He is best known for his book Anarchy, State, and Utopia , a right-libertarian answer to John Rawls's A Theory of Justice...
also joins McGinn in supporting this thesis.