The continuum of detection and awareness of visual stimuli within the blind field: from blindsight to the sighted-sight

Purpose. We investigated systematically the effect of repeated exposure on detection and reported awareness of visual stimuli presented deep within the field defect of 5 hemianopic patients.

Methods. An objective measure of sensitivity (detection in a temporal two-alternative forced-choice paradigm) and subjective reports of awareness were recorded on trial by trial bases. Visual stimulus to be detected was a temporally modulated (10 Hz) circular patch (6° diameter) of vertical grating (1 c/°). Hemianopic patients took part in the study 8 to 15 months after injury, so that the findings could not be attributed to spontaneous recovery.

Results. Initially, high contrast (90%) target stimuli were detected at or near chance level with little reported awareness. In 4 of 5 cases, repeated stimulation led to improved sensitivity, indicated by increased detection scores and higher incidence of awareness. In a fifth case, there was no change in sensitivity despite extensive exposure (>22,000 trials).
Conclusions. At retinal locations deep within the field defect, repeated stimulation can lead to blindsight performance (type I detection without awareness), followed by detection with reported awareness (type II blindsight), and eventual reported visual experiences. The findings indicate that conscious awareness of stimuli lies on a continuous spectrum and repeated systematic training can lead to improved visual sensitivity.