Blindsight colour perception opens up new questions about vision

For you to read these words, signals from your eyes must pass through a region of your brain called the primary visual cortex (also known as V1). Without V1, you'd be 'cortically blind' and you'd see nothing. But as classic research from the 1970s showed, some people with severe damage to V1, who have no awareness of seeing anything at all, can remarkably still respond to some visual stimuli. They might do better than chance at identifying whether a ball is appearing on their left or their right, for example — while denying that they can see any ball.  

This ability is known as 'blindsight'. A recent paper in Cerebral Cortex describes the case of a man who has it — but the exact nature of what 'TN' can 'see', and the fact that he apparently perceives colour, in particular, challenges traditional concepts of blindsight. Taken together, this case study suggests that we have more to learn about how people with healthy brains see the world.

At 52, TN suffered two strokes which damaged the cortex in both hemispheres of his brain, leaving him in what he described as "total darkness". But then came the first of two serendipitous discoveries. While one of the authors of the new paper was examining TN, he smiled and noticed that TN smiled, apparently in response. This was an example of a type of blindsight known as 'affective blindsight', which has previously been documented.

Then, three years later, while TN was being guided down a corridor, he was observed to move, apparently to avoid a collision. Follow-up evaluations revealed "a surprisingly well-preserved ability to avoid obstacles in his path" the team writes — despite TN's acknowledged loss of vision.

As a next step, they decided to explore whether TN might still perceive any colours. To do this, they watched him and questioned him as he tackled a series of challenges with differently coloured objects, including tulips, books, and Lego blocks.

In one challenge, for example, an experimenter put a block made up of green Lego pieces and another of red Lego pieces on a table in front of TN, and asked him to reach towards the red block. TN was adamant that this was impossible, as he couldn't even see any blocks. But when the experimenter asked him to try again, he looked down towards the table, said "it's here", and with a single motion, grasped the red block.

In some of the challenges, he would focus on an object for up to a minute, rotating it and holding it at various distances from his eyes, before reporting that the colour suddenly 'pierced' his eye. Identifying colours always took him some time and effort, the team notes. But while he wasn't able to pick out green objects, and blue was difficult, he repeatedly identified red items, in particular.

People with blindsight generally report not being able to see anything the researchers note. Even when they're shown evidence that they have some residual visual ability, as a rule, they are not convinced. In clear contrast, TN's words, along with his actions, suggest that he has some awareness of colour. "The fact that we got the same result using a variety of objects with different sizes and textures leads us to firmly believe that TN was — as he claimed — responding to colour as people generally understand it and as he himself did before the onset of his blindness," the team writes.

Given work showing that healthy brains can detect and even respond to emotional faces without conscious awareness of those faces, and that TN had an affective component to his blindsight, it's notable that he was much better at identifying red than any other colour. When someone is angry or embarrassed, their face can flush red. Red can, then, be among the cues to how another person is feeling — which, for our ancestors, may have had consequences for their survival.

There is a theory that survival-related processing can happen via two routes in the brain. One route is subcortical and allows for rapid responses without conscious awareness. (Supporting the idea of such a route, follow-up work on TN showed that emotional faces and direct gaze still activated the amygdala, a region of the brain activated by survival-related signals in the environment.) The second route is cortical — which in the case of visual data requires V1 — giving rise to conscious awareness.

However, TN's case suggests that the non-cortical route might allow for some level of conscious awareness of visual signals.

Given their findings, the team would now like to see re-evaluations of other people with blindsight. Standard testing for blindsight is formal and structured, and they think that it may not be as effective at identifying the types of residual visual ability that they found for TN. They'd also like to see further work to explore just what underpins his ability, as it may have implications for understanding not just blindsight but how people with healthy brains see the world.

Read the paper in full:
de Gelder, B., Humphrey, N., Pegna, A. J. (2024) On the bright side of blindsight. Considerations from new observations of awareness in a blindsight patient, Cerebral Cortex, bhae456, https://doi.org/10.1093/cercor/bhae456

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