IS BLINDSIGHT LIKE NORMAL, NEAR-THRESHOLD VISION

Blindsight is the rare and paradoxical ability of some human subjects with occipital lobe brain damage to discriminate unseen stimuli in the ir clinically blind field defects when forced-choice procedures are us ed, implying that lesions of striate cortex produce: a sharp dissociat ion between visual performance and visual awareness. Skeptics have arg ued that this is no different from the behavior of normal subjects al the lower limits of conscious vision, at which such dissociations coul d arise trivially by using different response criteria during clinical and forced-choice tests. PCP tested this claim explicitly by measurin g the sensitivity of a hemianopic patient independently of his respons e criterion in yes-no and forced-choice detection tasks with the same stimulus and found that, unlike normal controls, his sensitivity was s ignificantly higher during the forced-choice task, Thus, the dissociat ion by which blindsight is defined is not simply due to a difference i n the patients' response bias between the two paradigms. This result i mplies that blindsight is unlike normal, near-threshold vision and tha t information about the stimulus is processed in blindsighted patients in an unusual way.