Neurologist Oliver Sacks had a patient called D.F. with a form of visual agnosia. Visual agnosia refers to the inability recognise objects within your field of vision. D.F. could not visually recognise forms and objects but she could recognise objects by touch or smell. When given a card, she could not place it in a slot. But she managed to place it when she was told to insert it as if the object she had in her hand was a letter.
Seeing without seeing
Visual information is dependent on light. In order to receive visual information about an object, light rays need to hit that object and then hit us. The ray lights that hit us carry with them information about the object they previously hit.
D.F.’s case challenges our commonly held notion that in order to see an object all we need is to receive in our eyes light rays that previously hit the object. This is mainly how electronic devices record visual information work. But it does not seem to be the case for human brains.
The human brain possesses two different “routes” to deal with visual data. The ventral stream goes from the primary visual cortex to the temporal cortex and deals with the relatively slow process of constructing accurate models of the world. At the same time, the dorsal stream goes towards the parietal lobe and deals with visual-motor control in a quick manner. This means two things. First, we can carry out actions on objects using our vision before we can visually recognise those objects. Second, up to an extent, we can carry out actions on objects using our vision without having the ability to visually recognise those object.
Knowing this, D.F.’s case can be better understood as a case where a brain’s visual system has some impairment in its ventral stream. In other words, she can control objects without seeing them because visual-motor control precedes visual recognition of objects. That in itself is a very interesting fact.
Seeing what you don’t
Some patients with damage to the part of the visual cortex called V1 are known to respond to visual stimuli that they do not see due to their aforementioned damage.These patients are said to have blindsight.
These people have a blind area within their field of vision where they see nothing. When testing their visual perception on their blind area with visual stimuli, participants tend to react to stimuli even though they claim not to see it. From emotional reactions and eye tracking to “guessing” shapes and colours, they tend to be quite successful at matching their responses to the stimuli they do not see. Just like in the case of visual agnosia, visual information processing is not a serial process, rather, it is more like a parallel process where different regions receive the visual information and process it at the same time. While patients with blindsight have the V1 area damaged and cannot recognise objects visually, their alternative pathways are still intact and are likely to be the source of the visual abilities that blindsight display in spite of their apparent inability to see.
It seems that our will to act upon things we see is dependent on our ability to see those things. But then, what does that say about our proven ability to act upon things that our brain sees but we do not?