Is blindsight just degraded normal vision?
It is a conservative and reasonable suggestion that implicit functioning, as in blindsight, is simply a weakened, degraded form of normal functioning, especially as the parameters of vision in blindsight are themselves weakened--e.g., acuity is reduced, detection thresholds are raised, chromatic discrimination is coarser. But does this mean that maximum performance in blindsight is itself uniformly degraded? The answer is no, because there are many examples of very good performance in blindsight, in the absence of acknowledged awareness, especially if the visual parameters are chosen to be within a selective range. The question then is raised whether the parameters of blindsight vision are qualitatively similar to normal vision, or even to any manifestation of normal vision when it is uniformly degraded, for example, by threshold rises or injection of noise? Evidence is provided that blindsight vision and normal vision, even if it were degraded, are qualitatively different in certain respects. For example, there can be selective loss of colour but not of luminance contrast; visible after-images to unseen stimuli ("prime-sight") can be observed in one subject (DB); there can be super-sensitivity in the blind hemifield that is better than that of the intact hemifield; there can be a change in S-cone retinal inputs. Even when the stimulus contrast is deliberately lowered, blindsight performance itself does not degrade in parallel. Another meaning of "degraded" is that even when blindsight discriminatory or detection ability is excellent, the subject's acknowledged experience is as though the stimuli are weak and degraded to the point of extinction or near-extinction. But this is akin to the very meaning of blindsight itself, and sets the problem to be solved in neural systems terms and philosophical analysis rather than providing a conservative solution.
Weiskrantz
Larry
L
Department of Experimental Psychology, Oxford University, South Parks Road, Oxford, OX1 3UD, UK. larry.weiskrantz@psy.ox.ac.uk
D000222:Adaptation, Physiological / Q000502:physiology; D019575:Blindness, Cortical / Q000503:physiopathology*; D003117:Color Vision Defects / Q000209:etiology / Q000503:physiopathology; D006423:Hemianopsia / Q000473:pathology / Q000503:physiopathology*; D006801:Humans; D007088:Illusions / Q000502:physiology; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology; D014793:Visual Cortex / Q000293:injuries / Q000473:pathology / Q000503:physiopathology*; D014795:Visual Pathways / Q000293:injuries / Q000473:pathology / Q000503:physiopathology; D014796:Visual Perception / Q000502:physiology*