Abstract
The term blindsight describes the non-reflexive visual functions that remain or recover in fields of absolute cortical blindness. As visual stimuli confined to such fields are subjectively invisible, they are customarily announced by visible or audible cues that inform the patients when to respond. The pervasive use of cueing has spawned the widely held assumption that sight and blindsight differ in that only blindsight requires cueing. To test this assumption, we measured detection of auditorily cued and un-cued stimuli in three hemianopic patients. Stimuli fell onto the photosensitive retina of the subjectively blind field, onto the objectively blind optic disc, and, in one patient, into a region where they evoked impoverished conscious sight. Regardless of whether cues were given, performance was highly significant in the latter region of poor sight, clearly above chance in the subjectively blind field, and random in the optic disc control condition. Moreover, cues enhanced detection only in the relatively blind field. Showing that blindsight performance persists when cues are omitted, the results imply that non-reflexive responses can be initiated in the absence of both stimulus awareness and perceptible cues.
Highlights
Patients who suffered lesions of the primary visual cortex can detect, localize, and discriminate targets when ‘forced’ to ‘guess’ whether, where or which stimulus has briefly been presented to their cortically blind field (Weiskrantz, 1986; Stoerig and Cowey, 1997 for reviews)
PERFORMANCE IN THE UN-CUED CONDITION When the analysis was restricted to responses occurring in the response windows, detection performance again significantly exceeded chance level as long as stimuli were presented to the photosensitive retina of the blind field (Figure 2B)
These include (1) intra- or extraocular light scattered into the sighted hemifield, (2) eye movements towards the target positions, and (3) conservative responses criteria adopted when the blind field is tested (e.g. Campion et al, 1983)
Summary
Patients who suffered lesions of the primary visual cortex can detect, localize, and discriminate targets when ‘forced’ to ‘guess’ whether, where or which stimulus has briefly been presented to their cortically blind field (Weiskrantz, 1986; Stoerig and Cowey, 1997 for reviews) This counterintuitive ability to respond to subjectively invisible stimuli has become widely known as blindsight (Weiskrantz et al, 1974). Ever since Pöppel et al (1973) introduced forced-choice methods to study the non-reflexive residual visual functions that remain in the cortically blind fields of human patients, researchers have given visible or audible cues to inform the patients when to respond This customary cueing led to the conclusion that “Blindsight subjects have to be prompted or cued to give their better-than-chance “guesses”. This customary cueing led to the conclusion that “Blindsight subjects have to be prompted or cued to give their better-than-chance “guesses”. [...] Without such cues, the subject fails to respond” (Dennett, 1991, p. 328)
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