The Ferrier lecture, 1989. Outlooks for blindsight: explicit methodologies for implicit processes.

In primates the retina is connected with different targets in the brain via several parallel pathways, the largest of which is that going to the lateral geniculate nucleus of the thalamus and thence to the striate cortex, the geniculo-striate pathway. When this route is damaged in man, apparent blindness in a corresponding part of the visual field occurs, despite the integrity of the other parallel pathways. In animals, it has been demonstrated by conventional behavioural forced-choice techniques that extrastriate routes can sustain a variety of visual discriminations. Comparable discriminations are also possible in some human subjects with geniculo-striate damage when forced-choice 'guessing' techniques are used. 'Blind-sight' refers to those subjects who state that they are unaware of the visual stimuli, even when performing discriminations at high levels of proficiency. Extensions of this approach are reviewed, especially to spectral sensitivity and movement discrimination. But residual capacities can also be assessed without requiring guessing responses, thereby avoiding issues of differential response criteria and other practical difficulties. Effects of 'unseen' stimuli in the cortically blind field on the visible perception of concurrent stimuli in the intact field can be measured. Also, positive reactions of the autonomic nervous system, such as the galvanic skin response, can be recorded to visual stimuli presented in the blind field. Recent evidence demonstrates that the pupil in normal adult subjects is systematically sensitive to structural and chromatic features of visual stimuli. Pupillometry reveals specific changes and residual capacities in visual-field defects of adult patients with striate cortical damage. Thus non-verbal and sensitive methods are available that permit the comparative study of normal and residual visual capacity in human infants, adults and infra-human animals.