Blindsight
From Psy3242
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Definition
Blindsight is defined by the Oxford Concise Dictionary as "a condition in which the sufferer responds to visual stimuli without consciously perceiving them". Human patients with such a condition often have lost function of their primary visual cortex (V1) and claim they are blind. In primates and humans, the retina relays its major neural output to the thalamus and then to visual cortex (V1, or striate cortex). When the strait cortex is completely removed or blocked in the brain of a monkey, the animal can still discriminate between certain visual stimuli, though its overall capacity for normal functioning has changed. This is because the output from the eye also reaches various other brain regions in the midbrain and thalamus that often remain intact and undamaged when V1 is completely removed.
History
The history of blindsight stems from a focus on animal research and neuroanatomy, especially in the contrast of human and other primate visual cortex functions. One of the first hypotheses of residual vision in monkeys was made in 1886 by David Ferrier, who removed his subjects' entire visual cortexes and observed the 'blind' monkeys navigating around obstacles. More than a century and numerous studies later, the general consensus has been drawn that monkeys lacking a primary visual cortex can discriminate shapes, show sharp sensitivity to the detection of movement and contrast, and have measurable acuity. However, these abilities are discernibly reduced from normal functioning. In contrast to primates, evidence on the lack of V1 or damage to the area in human brains remained inconclusive until more recently.
An innovator in the field, Dr. Larry Weiskrantz came up with the oxymoron 'blindsight' at an Oxford University seminar when he was pressured for a title for his seminar on his striking behavioral research results, and the term began to be widely used in the 1970's.
The Case of DB
D.B. (blindsight patient) was a patient at the National Hospital in London who became the first major focus of research in human blindsight. His occipital lobe of his right hemisphere had been removed to alleviate headaches caused by a benign tumor in the area. While the surgery virtually cured him of his migraines, DB became blind in the left half of his visual field following the procedure. However, DB's 'blindness' was not normal in that he had retained an ability to locate objects and detect some changes in his blind visual field. Dr. Larry Weiskrantz of Oxford University conducted much of the research on the case, and instead of relying on DB's personal descriptions of what he couldn't see, he applied 'monkey-type' tests, or methods of forced-choice more commonly used with animals to measure DB's visual capacity.
What was so astonishing about DB was the accuracy of a variety of his discriminations by 'guesswork' in his blind visual field, even when he insisted he couldn't really 'see' anything. For example, he could tell if an object was in motion or stationary, the orientation of a pattern or grating, forced-choice guesses if there were 'lines' or 'no lines', and could even reach out and touch an object with a high, though not normal, level of accuracy - all without any kind of acknowledgment or awareness on his part. When shown his own results on many of the tasks he had performed, he remained disbelieving and attributed his success to chance. For ten years, DB was a focus of Weiskrantz's studies, and the resulting body of work was the book simply titled 'Blindsight' in 1986.
Dr. Larry Weiskrantz
Types of Blindsight
Varieties in the visual phenomenon range in the types of visual properties and attributes that can be successfully discriminated by the 'blind' individual. Changes in stimuli can include color, simple shapes, motion, different orientations of lines or gratings, onset or termination of visual events, to the emotional expressions on unseen faces. These emotional faces can be successfully identified through 'guessing' at much higher than chance levels.
However, there are severe changes in the abilities of a 'blindsight' patient from those visual abilities of a person with normal vision. Visual acuity - though credible - is severely reduced, and motion perception is limited from identifying more complex motion patterns. Color discrimination seems to be successful in patients, but there is often a shift in sensitivity toward long wavelengths (red) and away from short wavelengths (greens).
Interesting Link
For an interesting game that approximates an experience of a blindsight patient, go to this site: serendip.brynmawr.edu/bb/blindsight.html
