Transcranial magnetic stimulation induced blindsight : A systematic review

Abstract: Blindsight is a phenomenon in which patients suffering damage to the primary visual cortex (V1) perceive themselves as blind, but nonetheless seem to have some residual capacity to distinguish between visual stimuli better than chance. Blindsight can be divided into two subtypes: blindsight type I and blindsight type II. Blindsight type I is defined as visual capacity in the absence of acknowledged awareness. Patients with blindsight type II have visual capacity with some feeling or sensation in the blind field. Visual pathways bypassing V1 are assumed to be responsible for the residual capacity in blindsight. To investigate whether these pathways are present in healthy individuals we examined if it is possible to induce blindsight in healthy individuals by reviewing studies that have tried to induce blindsight with transcranial magnetic stimulation (TMS). We found that TMS might be able to induce blindsight type I of side detection. We also found that TMS might be able to induce blindsight type II of colour, orientation, and trustworthiness. Further, we found that both conscious and unconscious perception of shapes are dependent on processing in early visual cortex (EVC) in healthy individuals. We conclude that the full capacity seen in blindsight is most probable caused by neural reorganisation post trauma. The visual pathways bypassing V1 are, if present in healthy individuals, too weak to influence behaviour with the possible exception of side detection. Additionally, we conclude that the use of a binary awareness scale in blindsight studies fails to capture vaguely seen stimuli.