Blindsight

According to Zeki (1992, 1993), area V1 (the primary visual cortex) plays a central role in visual perception. Nearly all signals from the retina pass through this area before proceeding to the other areas specialised for different aspects of visual processing. Patients with partial or total damage of this area show a loss of vision in part or all of the visual field. However, in spite of this loss of conscious vision, some of these patients can make accurate judgements and discriminations about visual stimuli presented to the "blind" area. Such patients are said to show blindsight.

The most thoroughly studied patient with blindsight was DB, who was tested by Weiskrantz (e.g., 1986). DB's perceptual problems stemmed from an operation designed to reduce the number of severe migraines from which he suffered. Following the operation, DB was left with an area of blindness in the lower left quadrant of the visual field. However, he was able to detect whether or not a visual stimulus had been presented to the blind area, and he could also identify its location.

In spite of DB's performance, he seemed to have no conscious visual experience. According to Weiskrantz et al. (1974, p. 721), "When he was shown his results [by presenting them to the right visual field] he expressed surprise and insisted several times that he thought he was just 'guessing.' When he was shown a video film of his reaching and judging orientation of lines, he was openly astonished." However, it is hard to be sure that DB had no conscious visual experience, and the reports of other patients are sometimes confused on this issue. For example, patient EY "sensed a definite pinpoint of light", although "it does not actually look like a light. It looks like nothing at all" (Weiskrantz, 1980).

Weiskrantz, Barbur, and Sahraie (1995) argued that any residual conscious vision in blindsight patients is very different from conscious vision in normal individuals. They argued that it is characterised by "a contentless kind of awareness, a feeling of something happening, albeit not normal seeing" (Weiskrantz et al, 1995, p. 6122). They asked their patient to detect the direction of motion of a stimulus, and also to indicate whether he had any awareness of what was being presented. On "aware" trials, his detection performance tended to be better when the stimulus was moving faster. However, his performance on "unaware" trials did not depend on stimulus speed. As Weiskrantz (1995, p. 149) concluded, the patient's "unaware mode is not just a pale shadow of his aware mode".

Additional evidence that blindsight does not depend on conscious visual experience was reported by Rafal et al. (1990). They found that blindsight patients performed at chance when given the task of detecting a light presented to the blind area of the visual field. However, their speed of reaction to a light presented to the intact part of the visual field was slowed down when a light was presented to the blind area at the same time. Thus, a light that did not produce any conscious awareness nevertheless received sufficient processing to disrupt visual performance on another task.

Processing of motion Processing of colour Processing of form

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