Consciousness: Disorders

Blindsight

Blindsight refers to the unconscious detection of features that are not consciously perceived because of damage to the primary visual cortex. It was first reported in 1977 by Larry Weiskrantz. He collected evidence from a man who had undergone neurosurgery to remove a tumor from his primary visual cortex. The surgical procedure left the patient blind in the visual field opposite the affected hemisphere (contralateral). The primary visual cortex is the cortical entry point for most of the visual information received from the eye. It is located at the back of the brain with the right primary cortex receiving visual information from the left visual field (to the left side of fixation) and the left primary cortex receiving visual information from the right visual field (to the right side of fixation). The patient Weiskrantz studied lost his right primary visual cortex and saw nothing to the left of where he was currently looking, although, if the visual scene remained stable, he was able to compensate for his visual loss by moving his eyes. Thus, to probe his remaining visual abilities, he was tested by presenting stimuli briefly before he had time to move his eyes, thereby moving the stimulus into his good visual field.

Weiskrantz showed that after a light was briefly flashed in the patient's bad visual field, he was able to move his eyes to its previous location, or point at it much more accurately than if he had been guessing. Yet guessing is exactly what he reported he was doing; consciously, he saw nothing. Further testing revealed that he could discriminate the orientation of simple lines as well as the direction of moving dots above chance levels, although again, he denied seeing anything. Weiskrantz called this condition blindsight. At first, questions were raised, attributing the results, for example, to light scatter into good areas of the visual field, remaining pockets of the cortex, or undetected eye movements. However, in most patients, these alternative accounts have been ruled out.

Accepting that blindsight is a genuine phenomenon, what implications follow for the neural basis of consciousness? Does blindsight mean that the primary visual cortex is necessary for perceptual awareness of visual information? Other pathways for visual information to the cortex are known (e.g., superior colliculus to the frontal eye field; pulvinar and its connections to the posterior visual association cortex), but the fact that blindsight results from lesions of the primary visual cortex suggests that these other pathways are (at least initially) insufficient to support conscious visual awareness. One interesting question is whether other visual pathways can be trained to mediate conscious experience. Some patients with primary cortex lesions have shown spontaneous recovery of conscious vision, even after years of blindness, and monkeys, after full ablation of the primary visual cortex, have been trained to show improvements in visual discrimination. In a recent review, Tony Ro and Robert Rafal suggest that this evidence, at least in part, reflects learned access to visual awareness through pathways from the superior colliculus to cortical areas. Animals with lesions of both the visual cortex and the superior colliculus do not regain vision after training. These findings imply that visual awareness is at least partly a learnable skill. Normally, the salient information coming from the primary cortex dominates conscious visual experience, but, when that is removed, information normally used for other purposes (e.g., the planning of eye movements) can eventually create conscious experiences. One question that remains unanswered is the degree to which regained conscious experience is similar to normal conscious experience.

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