Cortical blindness · Grey Matter

Cortical blindness is the loss of sight caused by damage to the primary visual cortex of both occipital lobes, with eyes, optic nerves and pupillary reflexes that still work normally.

Cortical blindness. Cortical blindness is the loss of sight caused by damage to the primary visual cortex of both occipital lobes, with eyes, optic nerves and pupillary reflexes that still work normally.

Light still enters the eyes and the pupils still constrict, because the pupillary reflex runs through the midbrain and never needs the cortex, yet nothing reaches conscious vision because the visual cortex (Brodmann area 17) on both sides, or the optic radiations that feed it, has been destroyed. The usual cause is a stroke in both posterior cerebral arteries, which supply the occipital lobes; damage to one side alone gives homonymous hemianopia instead.

Some patients deny being blind. In Anton syndrome, described in 1895, a person with cortical blindness insists they can see, walks into furniture and confabulates descriptions of the room.

Some see without knowing it. In blindsight, patients who report seeing nothing can still point to, or detect the motion of, objects in the blind field above chance, through routes that bypass V1 (via the superior colliculus and the thalamus).

Mental imagery can survive it. The notes list lost visual imagery as a consequence of occipital damage, but patients have been described with cortical blindness or severe visual loss whose ability to imagine shapes, colours and faces was preserved, so perception and imagery partly rely on different circuits.

Questions: What can someone who is blind from damage to both visual cortices still do with their eyes? The pupils still react to light, because that reflex runs through the midbrain and never needs the cortex, and the eyes themselves are healthy. Some patients show blindsight: they report seeing nothing, yet point to a light or detect movement in the blind field above chance, through pathways that reach higher visual areas through the superior colliculus and the thalamus without passing V1. Others, in Anton syndrome, deny being blind at all and describe surroundings they cannot see, a failure of awareness added to the loss of vision.