Aware or unaware: assessment of cortical blindness in four men and a monkey

In four patients and one monkey with unilateral visual field defects caused by retro-geniculate lesions we measured forced-choice localization of square-wave gratings as a function of contrast, and compared results from the patients' absolutely and relatively blind fields. In addition, the patients indicated verbally whether they were aware of the stimuli. We then switched to a signal detection task in which the subjects had to signal a stimulus as in the localization task, by touching it, no matter whether it appeared in the good or bad hemifield, and in addition to signal a blank trial by touching an outlined square now constantly present on the monitor, and designated the no-stimulus response area. In this way, we could compare a non-verbal procedure that we had previously used in hemianopic monkeys with a verbal one commonly used to assess visual awareness. The results showed a close correspondence between the two measures of awareness in the human subjects who signalled 'stimulus' only for targets that also evoked verbal aware responses, validating the non-verbal approach. The hemianopic monkey behaved more like a patient with an absolute rather than a relative defect, and perfectly localized high-contrast stimuli which she nevertheless treated as blanks in the vast majority of presentations in the signal detection task.