Blindsight

Blindsight is the ability of people who are cortically blind due to lesions in their striate cortex, also known as primary visual cortex or V1, to respond to visual stimuli that they do not consciously see. The majority of studies on blindsight are conducted on patients who have the conscious blindness on only one side of their visual field. Following the destruction of the striate cortex, patients are asked to detect, localize, and discriminate amongst visual stimuli that are presented to their blind side, often in a forced-response or guessing situation, even though they do not consciously recognize the visual stimulus. Research shows that blind patients achieve a higher accuracy than would be expected from chance alone. Type 1 blindsight is the term given to this ability to guess—at levels significantly above chance—aspects of a visual stimulus (such as location or type of movement) without any conscious awareness of any stimuli. Type 2 blindsight occurs when patients claim to have a feeling that there has been a change within their blind area—e.g. movement—but that it was not a visual percept. Blindsight challenges the common belief that perceptions must enter consciousness to affect our behavior; showing that our behavior can be guided by sensory information of which we have no conscious awareness. It 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. Blindsight is the ability of people who are cortically blind due to lesions in their striate cortex, also known as primary visual cortex or V1, to respond to visual stimuli that they do not consciously see. The majority of studies on blindsight are conducted on patients who have the conscious blindness on only one side of their visual field. Following the destruction of the striate cortex, patients are asked to detect, localize, and discriminate amongst visual stimuli that are presented to their blind side, often in a forced-response or guessing situation, even though they do not consciously recognize the visual stimulus. Research shows that blind patients achieve a higher accuracy than would be expected from chance alone. Type 1 blindsight is the term given to this ability to guess—at levels significantly above chance—aspects of a visual stimulus (such as location or type of movement) without any conscious awareness of any stimuli. Type 2 blindsight occurs when patients claim to have a feeling that there has been a change within their blind area—e.g. movement—but that it was not a visual percept. Blindsight challenges the common belief that perceptions must enter consciousness to affect our behavior; showing that our behavior can be guided by sensory information of which we have no conscious awareness. It 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. We owe much of our current understanding of blindsight to early experiments on monkeys. One monkey in particular, Helen, could be considered the 'star monkey in visual research' because she was the original blindsight subject. Helen was a macaque monkey that had been decorticated; specifically, her primary visual cortex (V1) was completely removed, blinding her. Nevertheless, under certain specific situations, Helen exhibited sighted behavior. Her pupils would dilate and she would blink at stimuli that threatened her eyes. Furthermore, under certain experimental conditions, she could detect a variety of visual stimuli, such as the presence and location of objects, as well as shape, pattern, orientation, motion, and color. In many cases, she was able to navigate her environment and interact with objects as if she were sighted. A similar phenomenon was also discovered in humans. Subjects who had suffered damage to their visual cortices due to accidents or strokes reported partial or total blindness. In spite of this, when they were prompted they could 'guess' with above-average accuracy about the presence and details of objects, much like the animal subjects, and they could even catch objects that were tossed at them. The subjects never developed any kind of confidence in their abilities. Even when told of their successes, they would not begin to spontaneously make 'guesses' about objects, but instead still required prompting. Furthermore, blindsight subjects rarely express the amazement about their abilities that sighted people would expect them to express. Patients with blindsight have damage to the visual system that allows perception (the visual cortex of the brain and some of the nerve fibers that bring information to it from the eyes) rather than the system that controls eye movements. This phenomenon shows how, after the more complex visual system is damaged, people can use the latter visual system of their brains to guide hand movements towards an object even though they cannot see what they are reaching for. Hence, visual information can control behavior without producing a conscious sensation. This ability of those with blindsight to 'see' objects that they are unconscious of suggests that consciousness is not a general property of all parts of the brain; yet it suggests that only certain parts of the brain play a special role in consciousness. Blindsight patients show awareness of single visual features, such as edges and motion, but cannot gain a holistic visual percept. This suggests that perceptual awareness is modular and that—in sighted individuals—there is a 'binding process that unifies all information into a whole percept', which is interrupted in patients with such conditions as blindsight and visual agnosia. Therefore, object identification and object recognition are thought to be separate processes and occur in different areas of the brain, working independently from one another. The modular theory of object perception and integration would account for the 'hidden perception' experienced in blindsight patients. Research has shown that visual stimuli with the single visual features of sharp borders, sharp onset/offset times, motion, and low spatial frequency contribute to, but are not strictly necessary for, an object's salience in blindsight. There are three theories for the explanation of blindsight. The first states that after damage to area V1, other branches of the optic nerve deliver visual information to the superior colliculus and several other areas, including parts of the cerebral cortex. In turn, these areas might then control the blindsight responses. Another explanation for the phenomenon of blindsight is that even though the majority of a person's visual cortex may be damaged, tiny islands of functioning tissue remain. These islands aren't large enough to provide conscious perception, but nevertheless enough for some unconscious visual perception. A third theory is that the information required to determine the distance to and velocity of an object in object space is determined by the lateral geniculate nucleus before the information is projected to the visual cortex. In a normal subject, these signals are used to merge the information from the eyes into a three-dimensional representation (which includes the position and velocity of individual objects relative to the organism), extract a vergence signal to benefit the precision (previously auxiliary) optical system, and extract a focus control signal for the lenses of the eyes. The stereoscopic information is attached to the object information passed to the visual cortex. Evidence of blindsight can be indirectly observed in children as young as two months, although there is difficulty in determining the type in a patient who is not old enough to answer questions.