Extrageniculate mediation of unconscious vision in transcranial magnetic stimulation-induced blindsight

Abstract

The proposed neural mechanisms supporting blindsight, the above-chance performance of cortically blind patients on forced-choice visual discrimination tasks, are controversial. In this article, we show that although subjects were unable to perceive foveally presented visual stimuli when transcranial magnetic stimulation over the visual cortex induced a scotoma, responses nonetheless were delayed significantly by these unconscious distractors in a directed saccade but not in an indirect manual response task. These results suggest that the superior colliculus, which is involved with sensory encoding as well as with the generation of saccadic eye movements, is mediating the unconscious processing of the transcranial magnetic stimulation-suppressed distractors and implicate a role of the retinotectal pathway in many blindsight phenomena.

Fig. 1.

The stimuli and timing used in this experiment. Distractors, when presented on 50% of the trials, were slightly offset to the right of fixation in each subject because this was the location that produced the most consistent and robust visual suppression with TMS, likely because of a further posterior extension of the left occipital lobe in humans (29).

Fig. 2.

The mean saccadic latencies for the TMS/unaware and no TMS/aware conditions (a) and the mean manual latencies for the TMS/unaware and no TMS/aware conditions (b). Note the change in scale in b to accommodate for the overall slower responses and larger difference on the aware trials in this task. Error bars represent one standard error of the mean. ^*, P < 0.02; ns, not significant.