Blindsight revisited

Some human patients with lesions to their primary visual (striate) cortex (V1) demonstrate residual visual capacity, but without acknowledged perceptual awareness. This phenomenon has been termed blindsight. Recent results from work on blindsight patients suggest that it is unlikely to be attributable to intact residual areas (tags) of V1. Previous research has reported that blindsight patients can retain the ability to detect monochromatic light and grating stimuli, and to discriminate orientation and direction of movement in their 'blind' fields. These findings have been joined by reports that these patients also are sensitive to, and are able to discriminate, wavelength in the absence of any experience of 'colour'. This reveals that retinal pathways other than those to the striate cortex are crucially involved in vision. Conditions can be controlled for obtaining either acknowledged awareness or unawareness of discrimination of the direction of a small moving target in blindsight patients. This potentially offers the possibility to determine whether there are structures uniquely involved in visual awareness. Monkeys lacking V1 also clearly demonstrate residual visual capacity, and some evidence exists that they also experience 'blindsight'.