The sight that isn't there
Blindsight is a condition where individuals with damage to the brain's primary visual cortex (V1) report being unable to see, yet can respond to visual information. This paradoxical ability was systematically documented through research heavily associated with the University of Oxford. The term "blindsight" was coined by neuropsychologist Lawrence Weiskrantz and his colleagues in 1974 following work with a patient known as D.B.
Patient D.B. had his right visual cortex surgically removed to treat a tumor, resulting in blindness in the left half of his visual field. Despite his insistence that he saw nothing, Weiskrantz's experiments showed D.B. could accurately "guess" the location of a point of light more than 80% of the time. He could also differentiate between simple shapes, like an 'X' and an 'O', and determine the orientation of lines, all without any conscious visual experience. This split between visual processing and conscious awareness opened a new field of study into the brain's multiple visual systems.
The brain's ancient backroad
Conscious vision relies on the primary visual pathway, where information travels from the retina to the lateral geniculate nucleus (LGN) of the thalamus, and then to the V1 cortex at the back of the brain. Damage to V1 disrupts this flow, causing cortical blindness. Blindsight shows that this is not the only route for visual data.
The phenomenon occurs because of secondary, more primitive visual pathways that bypass the V1 cortex. One important route is the retinotectal pathway. In this system, signals travel from the retina to a midbrain structure called the superior colliculus, then on to other brain regions like the pulvinar nucleus of the thalamus and into the extrastriate cortex—higher-level visual processing areas. This pathway does not create a conscious image but carries basic information about location, motion, and general form.
Another patient extensively studied at Oxford, G.Y., lost his left V1 due to a traffic accident at age 8. His case shows evidence for these alternative pathways. G.Y. is good at detecting fast-moving stimuli in his blind field, an ability attributed to a direct connection from the LGN to the motion-processing area V5/MT, bypassing V1 completely.