Blindness refers to the situation that some people turn a blind eye to a certain area in their field of vision. Type I blind people claim that they can't see anything, but their judgment on the position or movement type of objects in this area is much higher than random guess. Type II blindness is different. The patient claims that he can feel the motion information of some objects, but he has no visual perception. Blindness is caused by damage to a specific area of the visual cortex (see occipital lobe).

Visual information processing in the brain requires a series of steps. The damage of the primary visual cortex will lead to the loss of vision in the corresponding visual field. The area of vision loss in visual field, also called blind spot, varies according to the damage range, which can be very small or as large as the whole half of visual field.