Seeing the invisible: theory and evidence of blindsight

Since the fifties of the last century, a bulk of research demonstrated that unconscious visual functions may persist in the blind hemifield after stroke, striate deafferentiation, or ablation. This experience is known as blindsight. Great effort has been paid to clarify the mechanisms underlying this fascinating phenomenon: the characteristic dissociation in performance between yes/no response design and alternative forced choice procedures, the relationship between conscious and unconscious perception, the anatomo-functional substrates, and, last but not least, the implications within the clinical setting. In this review, the theoretical and experimental evidence of blindsight are described. Bibliographic research was carried out on PubMed, Google Scholar and other resources supporting biomedical literature, starting with the following keywords (isolated or in combination): blindsight, unconscious vision, hemianopia, stroke, extrastriate pathway. The references in each selected paper were then analyzed. Overall almost 200 studies were examined and approximately 150 of them were considered suitable for our purpose. The main findings are reported and organized into two sections: “theory and evidence of blindsight” and “testing blindsight”. According to the current theories, blindsight may be degraded conscious vision or it can rely on extrastriate routes. It encompasses a large number of tasks such as grating, color, orientation detection and discrimination, contrast sensitivity and implicit processing, and is believed to rely on retinal-extrastriate routes that bypass V1. Finally, blindsight can be trained. Blindsight is worth being investigated in hemianopic patients suffering from stroke or striate deafferentiation not only within a theoretical framework but also from a rehabilitative perspective.