Blindsight: The seeing blind

Even though your eyes are neccessary for you to see, they are not sufficient. For conscious vision, you also need the visual areas at the back of your brain. People with damaged visual cortex are cortically blind, even though their eyes are intact.

Look at this remarkable video below. Here you see a man (called TN) navigating his way through a long hallway full of obstacles, cautiously, but very precisely. control-out of4This is no small feat for him, as he was left totally blind across the whole visual field, as a result of two strokes that damaged his primary visual cortex in both hemispheres.

This is kind of amazing, is it not? The man is not conscious of seeing anything around him and yet he can accurately avoid the obstacles in his way! If I didn’t know better, I would probably attribute his accomplishment to a miracle, extrasensory perception or some other mystical wonder … but as it happens … I do know better.

What you see in this video is an example of what is called blindsight. Blindsight is defined as the ability of people who are cortically blind due to lesions in their striate cortex, also known as the primary visual cortex or V1, to respond to visual stimuli that they are not aware of seeing.

How can this be?

Blindsight has been explained by two separate visual pathways in the brain that evolved at different times and that serve different aspects of vision. The first one is evolutionary older and more primitive. It goes from the eyes to the brainstem and is relayed from the there to higher centers of the brain. This pathway is concerned with reflexive behavior, for example orienting our attention to sudden movements in our periphery, controlling eye movements and directing gaze and head. The second one, an evolutionary newer and more sophisticated pathway, goes from the eye, through the thalamus, to the visual cortex. This pathway is thought to help us perceive the world around us consciously.

In people with blindsight, the newer visual pathway is damaged, but the older is intact. Because of damage to the primary visual cortex, the patient doesn’t perceive consciously anything that falls within his or her visual field. Here the emphasis is on the word “consciously”, because as is obvious from the video of TN, there seems to be another kind of vision, some kind of perception that guides him through space, without him being aware of it. TN himself claimed he was not aware of any objects in his way, he just walked as he did because he felt like doing so.

What was helping TN navigating the hallway was his more primitive visual pathway, which was left intact by his strokes.

Is my will as free as I would like to think?

As if all the above isn’t exciting enough, this phenomenon has other implications. One is that sensory information we are unaware of, is certainly able to affect our behavior.

This begs the question: To what extent are my actions influenced by things that I am not aware of?
Well … I am not aware of what I am not aware of perceiving, so I don’t have a clue!

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