Abstract
Blindsight has been widely investigated and its properties documented. One property still debated and contested is the puzzling absence of phenomenal visual percepts of visual stimuli that can be detected with perfect accuracy. We investigated the possibility that phenomenal visual percepts of exogenous visual stimuli in patient GY might be induced by using transcranial direct current stimulation. High contrast and low contrast stimuli were presented as a moving grating in his blind hemifield. When left area MT/V5 was anodally stimulated during the presentation of high-contrast gratings, he never reported a phenomenal percept of a moving grating but showed perfect blindsight performance. When applied along with low contrast gratings, for which accuracy was titrated to 60–70 %, performance did not improve but responses were significantly faster. Cathodal stimulation had no effect. Results are explained in the framework of GY’s reorganized cortical connexions and oscillatory patterns known to be involved in awareness in GY. The apparent presence of phenomenal visual percepts in earlier studies is shown to be a semantic confusion about what he means when he says that he sees in his blind field.
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Acknowledgments
AC was supported by a Leverhulme Emeritus Fellowship and IA by a grant from the EPA Cephalosporin Research Fund, Oxford. AE was supported by a grant from the Dr. Hadwen Trust for Humane Research. We thank subject GY for his continuing interest in blindsight and his willingness to describe his experiences and submit to lengthy psychophysical testing while being stimulated with tDCS.
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Cowey, A., Alexander, I. & Ellison, A. Modulation of cortical excitability can speed up blindsight but not improve it. Exp Brain Res 224, 469–475 (2013). https://doi.org/10.1007/s00221-012-3327-x
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DOI: https://doi.org/10.1007/s00221-012-3327-x