Visual evoked potentials in the investigation of "blindsight"

We recorded long-latency visual evoked potentials in four patients with homonymous hemianopias, one of whom had clinical evidence of "blindsight." Stimuli consisted of different words that appeared randomly and at a constant angle to either side of the center of a TV screen, and subjects responded to one previously specified word (the "target") by finger extension. Target stimuli in the intact hemifield elicited a well-formed P3 response in all subjects, whereas stimuli in the blind field produced no such response except in the subject with blindsight. In addition, the earlier potentials in this subject were larger with stimulation of the blind hemifield than the intact field. By contrast, a P100 response was present only with stimulation of the intact field in this subject. These results indicate that cognitive processing of visual stimuli can occur even when subjective awareness of such stimuli is absent, and suggest that such processing occurs independently of the geniculostriate pathway.