Experiments show that people who are blind as a result of a stroke or traumatic brain injury continue to see. They avoid obstacles in the corridor, recognize the emotions on the face of the interlocutor and guess what is shown in the pictures. Perhaps the same thing helps them, thanks to which a person reacts to an approaching danger.
Eyes watch, brain sees
The retina of the eye perceives light coming from a source or reflected from objects. This information travels to the thalamus, a part of the brain responsible for transmitting sensory and motor data from the senses. From there - to the primary visual (striate) cortex, which separates static and moving objects, recognizes images.
Then - into the secondary, or extrastriatal, visual cortex. And from there, to the associative zones of the brain, where the final recognition of objects takes place and a reaction to them is formed.
If the primary visual cortex is excluded from this chain - namely, it can suffer from a stroke or traumatic brain injury, the person actually goes blind. His eyes are healthy and continue to see, but his brain does not respond. However, there are exceptions.
Visual information from the retina goes first to the thalamus (the figure shows the lateral geniculate nucleus included in it), and from there to the primary visual (striate) cortex. It separates static and moving objects, recognizes images. The processed information then goes to the secondary visual cortex. And from there - to the associative areas of the brain, where the final recognition of objects takes place.
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Guessing too accurate
The Dutch and British scientists described two cases at once when patients who lost their sight after a head injury correctly recognized the emotions of the people depicted in the pictures. The volunteers did not have time to answer whether the person in the photo was afraid or happy, but their brain already knew the correct answer.
Electrodes were attached to the volunteers' faces, recording nerve signals that were looking for muscles that tense when a person smiles or, conversely, frowns. It turned out that the volunteers copied the expressions of the faces depicted in the pictures, although they claimed that they could not see anything. Moreover, their primary visual cortex showed no signs of activity.
Similar was the case with a 50-year-old man who lost his sight after a second stroke. During the experiment - he also looked at photographs of faces - he was placed in an fMRI scanner that measured brain activity. It turned out that when the patient looked at images of people looking at him point-blank, the amygdala, the cerebellar amygdala, which is responsible for processing emotions reflected on the faces of others, was activated.
True, the participant in the experiment himself, guessing whether the person in the picture is looking at him or not, was not mistaken with only half of the photographs, that is, he did not go beyond the bounds of chance. On the other hand, another patient with a damaged primary visual cortex guessed the objects depicted on the screen with an accuracy of 90 percent. Moreover, he claimed that he did not see anything, and the correct answers were just luck.
Workaround
A real sensation was made by a patient who is called TN in the scientific literature. He went blind after a stroke and walked with a cane. The scientists took her away from him and asked him to walk down the corridor with boxes and chairs scattered about. TN did an excellent job on the first try, avoiding all the obstacles without much difficulty.
As the authors of the work note, the subject was not even aware that he was bypassing objects: "He found it difficult to explain or at least describe his actions." Moreover, he claimed that he was just walking straight along the corridor.
According to Dutch and Swiss scientists, this is possible due to the fact that the functions of the inoperative primary visual cortex are taken over by the tubercles of the midbrain quadruple - structures that also specialize in processing visual information.
The brain of a patient blinded after a stroke. Damages in the primary visual cortex are shown in dark color. In experiments, this patient, despite his blindness, predicted by the size of the figure when the sound would increase.
The fact is that the lower tubercles are usually responsible for processing sound stimuli, and in the upper ones, part of the optic nerve fibers ends and fast processing of data received from the retina occurs. This allows you to escape from an approaching threat - for example, a predator - even before the body realizes what is happening. From the superior tubercles of the quadruple, information enters the thalamus, and then immediately into the secondary visual cortex.
This appears to persist in patients with damage to the primary visual cortex. Therefore, they distinguish between faces, they are able to bend around obstacles.
Moreover, complex visual-auditory associations are formed when a blind person correlates sound with the size of an object. The researchers asked a volunteer with a damaged striatal cortex to press a button if he thought the sounds should increase. There was a red circle on the screen in front of him, which sharply decreased before the volume was turned up. The blind man pressed the button more and more rapidly as he squeezed the circle. This means that a causal relationship arose in his brain between the volume of the sound and the size of the figure, although he did not see it.
The authors of the work believe that thanks to this mechanism, people who are blinded by trauma can recover some visual skills and even learn something new.