We had Tony, one of our listeners in touch about a question in our recent QnA. One of our experts Keziah Latham was talking about what blind people actually see, especially give that 95% of people who are legally blind still have some visual perception. But Tony wanted to know more. Say they were born blind but with a working visual corext in the brain. Well, we went back to Keziah, who said...
Chris - The questioner is right to suggest that processing of vision in the eye and in the brain is not always exactly the same. One example of this is that it is relatively common for people with acquired sight loss (due to ocular problems, but with a working visual cortex) to experience visual hallucinations in their non-seeing areas of vision. This is called Charles Bonnet Syndrome. It is thought to be due to visual cortex being ‘bored’ by no longer receiving input from the eyes, and neural excitation creating images that the person can see. These hallucinations can be upsetting to people, and anyone who experiences visual hallucinations related to sight loss is encouraged to contact Esme’s Umbrella (https://www.charlesbonnetsyndrome.uk/) for advice and support.
Adam - On the other hand, some people who are blind due to cortical problems, but have no problems with the eyes themselves, appear to be able to respond to visual stimuli that they do not consciously see, which is termed ‘blindsight’. This can occur after stroke, which typically affects one side of the brain and thus one side of vision. People with blindsight can respond to stimuli presented in their blind field with an accuracy greater than chance, despite being unaware of seeing any visual stimuli. This suggests that some visual processing bypasses the usual visual pathway to the occipital cortex.
However, if someone is born with no sight, their occipital cortex will not develop the ability to ‘see’ – input from the eye to the brain is needed in the first years of life to fine tune visual processing ability through experience. If this is lacking, then the eye / pathway becomes amblyopic or ‘lazy’.
Chris - However, Tony Morland at York University has shown that there is some plasticity in the visual cortex, in that people born with only one type of photoreceptor in the eye can show signs of remapping of the visual cortex in compensation. This plasticity does not seem to occur if loss occurs later in life.