There are some people that can respond to visual stimuli even though they are blind. They can’t consciously see, yet they can unconsciously avoid obstacles. This phenomenon is called blindsight. The people who have it have had lesions to their primary visual cortex, rendering them blind. However, even though they can’t see, they can guess what it’s in front of them at a much higher accuracy than chance.
This phenomenon was first observed in monkeys. Even though they were missing their primary visual cortex their pupils dilated and their eyes blinked when there were threats to their eyes. They could not consciously see the threat, but their body gave an automated response. One particular monkey was also able to distinguish objects and shapes and colours.
“In the 8 years between the operation and her death in 1973 this monkey slowly recovered the use of her eyes, emerging from virtual sightlessness to a state of visual competence where she was able to move deftly through a room full of obstacles and could reach out and catch a passing fly” (Humphrey, 1974).
Even though the main pathway of visual signals is through the primary cortex, the eye does send visual information to other parts of the brain.
Basically, it is hypothesised that mammals have two ‘brain systems’. The first is quite primitive and similar to that of fish or reptiles. The second is much more complex and exclusive to mammals. It is this second system that gives us consciousness. In patients that have blindsight, the second system is disrupted, but the first one is still intact. Which is why they can still react to visual stimuli even though they can’t consciously see. It is the first system that guides them past objects.
Avoiding obstacles you can’t see
TN was a famous case study. His visual cortex had been damaged by two strokes. Researchers had him walk down a corridor filled with obstacles but did not tell him that the obstacles were there. The man easily avoided the obstacles and at some point even squeezed in between a trash can and the wall. When asked about it afterwards he said he was not even aware that the obstacles were there. In his mind, all he did was walk down an empty corridor. Because it was his non-conscious brain guiding him, he was simply not aware of it. You can see him in the video below:
TN was also able to recognise facial expressions. Brain scans showed increased amygdala activity when he was presented with faces with various emotions.
Surprisingly, while some people with blindsight can recognise emotions, they can’t recognise the person or the gender of the person.
Another patient, in a recent study, was able to grasp different sized objects in front of her, adjusting her grip every time. However, when asked, she was not aware of how big the objects were.
Thinking you can see
Blindsight could also be thought of as the opposite of Anton-Babinski syndrome. Patients who have it are blind, but think they can see. No matter how much proof there is that they can’t see, they will still adamantly affirm that they can. It can be caused by a stroke, like blindsight. We are not certain why patients cannot admit that they can’t see. An hypothesis is that the link between the speech-areas of the brain and the, damaged, visual cortex is broken. The visual information is received but cannot be interpreted by the brain, so it makes it up.
- Humphrey, N. (1974). Vision in a monkey without striate cortex: A case study. Perception
- Whitwell RL, Striemer CL, Nicolle DA, Goodale MA. (2011). “Grasping the non-conscious: preserved grip scaling to unseen objects for immediate but not delayed grasping following a unilateral lesion to primary visual cortex.”. Vision Res. 51 (8): 908–24. doi:10.1016/j.visres.2011.02.005. PMID 21324336
- Blind man ‘sees’ his way past obstacles – New Scientist
- Blindsight: Seeing without knowing it – Scientific American
- Prigatano, George P.; Schacter, Daniel L (1991). Awareness of deficit after brain injury: clinical and theoretical issues
- Blindsight – Scholarpedia