D.B. (blindsight patient)
From Psy3241
Contents |
Blindsight
· A condition in which the sufferer responds to visual stimuli without consciously perceiving them
· Historical origins stem from animal research
· The primate retina sends its neural output to through the thalamus and to the visual cortex area labeled V1 or striate cortex
· When the striate cortex is removed in animal studies, the animals still carry out visual discriminations; however their capacity changes
· Subjects are able to still carry out visual discrimination because the output from the eye also reaches other brain targets in the midbrain and thalamus that remain intact when the V1 is removed
· Human subjects that suffer a loss of V1 claim to be blind, even though they can carry out visual discrimination tasks
· The secondary routes used when the primary visual cortex has been removed are not sufficient to maintain the feeling of sight, which could explain why blindsight patients have the subjective feeling that he or she is blind, but can report visual information when forced to guess
History
· Originally discovered when trying to compare the visual cortex between monkeys and humans
· Recently, it has become clear that monkeys without primary visual cortex can discriminate shapes, demonstrate contrast sensitivity functions over a range of spatial frequencies, and have measurable acuity that is reduced from normal
· It has also been shown that these monkeys have remarkable sensitivity to small and brief visual events and will fixate on them or reach for them
· Monkeys with blindsight are also sensitive to the detection of movement
· Human subjects are blind in comparison to monkeys when the primary visual cortex is removed
· Early neurologists, Poppelreuter and Riddoch, studied World War I victims and claimed that there could be some residual function
· Teuber and colleagues supported this claim when studying World War II victims
· Progress in the comparison of monkeys to humans was made when researchers began studying humans in the same manner as monkeys by having them reach out to touch briefly presented stimuli
· In 1973 Held and Frost completed this test on subjects with their visual cortex removed because they believed that midbrain pathways were implicated in the saccadic control, which survived when visual cortex was removed
· Even though the subjects claimed that they could not see the stimuli in their field defects, there was a significant correlation between stimulus location and target eye position
D.B.
The First Studied Blindsight Patient
· D.B.’s primary visual cortex was removed surgically from his right hemisphere to ablate a benign tumor that had invaded the area
· The primary visual cortex relays visual information from the eyes to higher coritcal visual areas
· As a result of the removal of his right primary visual cortex, D.B. became blind in his left visual field
· He was tested in a way similar to animal testing by being told to guess at answers whether or not he can see them
· Termed forced-choice testing
· The result of this testing in D.B was that he could succeed ina variety of discriminations tasks by guessing in his blind field, even though he said he did not see in his blind field
· He could tell the direction a grating was oriented and whether a stimulus was moving or stationary
· His visual acuity was tested by varying the spacing of a grating and forcing D.B. to guess whether there were lines or no lines
· D.B.’s ability to reach out and locate the position of a stimuli in his blind field was good, but not normal
· This ability matched the ability of monkeys tested without primary visual cortex
· D.B. had a high level of visual proficiency in making discriminations in the absence of being aware of a stimuli
· Weiskrantz and Warrington completed the testing on D.B. begning in the 1970s and the testing continues today
· When the researchers informed D.B. of his test results indicating his impressive performance without awareness, he was astonished because he thought he could not see in his blind field
· D.B. remained convinced that he was simply guessing and that his performance was based on chance
· D.B. was studied by Weiskrantz and Warrington for over 10 years
· In 1986 the two researchers published a book titled “Blindsight” regarding D.B.’s capacity to detect and discriminate orientation, movement, and form under a variety of conditions
· D.B.’s basic phenomenon of “unconscious” visual capacity remains; however, his sensitivity has improved
Results of Blindsight Studies
· The first results obtained by Weiskrantz and Warrington were published in 1974
· The term “blindsight” was created as a result of Weiskrantz having to respond urgently to produce a title for a seminar he was invited to present at regarding D.B.’s phenomenon
· The first occurance of the term “blindsight” in print occurred in 1974 in a short article by Sanders, et. al. describing the phenomenon of visual function in a field defect
· The term stuck and now appears in dictionaries and papers throughout the globe
· It was also discovered that rapid events such as rapid movement or the sudden onset of an object produced a “feeling” that some event had occurred even though it was not seen, which became known as Blidsight Type 2
· Blindsight Type 1 is the situation in which there is no reported experience
· Recently it has become evident that this phenomenon is not rare
