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There are some people who claim to be unable to see things, and the results of vision examinations show that they have gone blind, but they can subconsciously guess the location and shape of the objects in front of them and avoid obstacles on the road. What's going on here?
The discovery of blindness
At first glance, "blindness" seems to be a contradictory term, but since it is "blind", how can we still talk about "blindness"?
Although the term "blindness" was coined in the 1970s, it has long been discovered. At the end of the 19th century, hermann munk, a physiologist in Berlin, Germany, conducted an experiment on the destruction of the occipital cortex of dogs and monkeys while arguing with the British physiologist David Ferrier about where the visual center of the cerebral cortex was. He found that damage to the pillow lobes could lead to what he called "mind-blindness." These dogs can still "feel" something, avoid or jump over obstacles, but they just can't recognize what it is. But because dogs can't speak, there's no way to know if they're seeing something, or if they see it but don't recognize it.
Wounded soldiers of the First World War brought clues of blindness. In 1917, The British physician George Riddoch and the German surgeon Walter Poppelreuter independently discovered that some brain-wounded soldiers had large blind spots in their field of vision. One of the most famous wounded soldiers Dr. Riddock treated was Major T, who was shot through the right occipital lobe by a bullet and fought for 15 minutes before remaining in a coma for 11 days. He woke up to find himself unable to see the food on the left side of the plate, because signals from the left visual field reached the cerebral cortex, and the first stop was in the right occipital lobe. When he returned to England, he found that although he could not see what was in the left field of view, he could perceive that there was no object moving in this field of vision. As he rode the train, he could feel something moving rapidly on the left side of the field of vision, but he couldn't see exactly what it was. He was troubled by this.
At the time, however, scientists focused their interest on where the visual center of the cortex was, and the position of objects in the field of vision and their correspondence in the visual system projection, without paying much attention to these strange phenomena and without studying them in depth.
[Invisible] but always able to [guess right]
The official scientific study of "blind vision" and the first to propose the term was the British neuroscientist Lawrence Weiskrantz. In the early 1970s, he discovered a patient, D. b. 。 34-year-old .b. is a computer programmer with a home, a love of music, and a happy life. But the weather is unpredictable. Initially, he had a headache, but after being diagnosed and treated by a neurologist, he found that a large tumor had grown in the primary visual cortex in the right hemisphere of his brain. When the tumor was surgically removed, a large piece of the right primary visual cortex had to be removed, leaving a large blind spot in his left visual field. If the object is displayed in the corresponding area, d .b will say that he cannot see. However, his left primary visual cortex is intact, so he can see the right half of the field of vision. Since he could turn his head and body, as well as his eyeballs, this kind of heelism didn't cause much of a problem in his life. Nevertheless, he agreed to continue working with Weiskrontz on the phenomenon of helinding.
When you put the object in d. b. In the right half of the field of vision, he can see these things and can say what they are. But if you put it in the blind spot of his left half of the field of vision, he says he can't see it. Curiously, however, when WeissKrantz showed either a square or a diamond in his blind spot and insisted that he guessed what it was, he was able to guess eight or nine. If he had to guess whether the object in the blind spot was moving, he guessed correctly much more often than randomly. By putting an object in his blind spot and asking him to point to the location with his finger or simply pick it up, he could accomplish the task quite well—even though he said he didn't see anything. If there is a sudden change or rapid movement in his blind spot, he can also feel as if something is happening, but still says nothing to see. When Weiskrontz said he was doing a good job, he couldn't believe it himself, insisting that he was just guessing casually. That is to say, in the sense of whether he is aware of seeing an object, d.b. does not see, so he is "blind"; but from the point of view of subconscious visual response, d .b. has received visual information to some extent, and can also use it to guide his actions, so that he can "see". Thus, to call this phenomenon "blindness" is not only not contradictory, but should also be said to be extremely appropriate.
Two visual pathways, old and new
So why is there such a strange phenomenon of blindness? Why don't the blind people we usually see have such magical abilities? This starts with the two visual pathways in the mind, old and new. The "new" and "old" here are in evolutionary order.
The photoreceptors in the visual system are all on the posterior wall of the retina of the eye. Photoreceptors receive light stimuli from the outside world, convert light signals into electrical signals, and after a series of processes, transmit them from the optic nerve to the brain. The optic nerve emitted from each side of the retina is divided into two strands, the optic nerve emitted from the side near the temple (temporal side) has been ascending in the same cerebral hemisphere, while the optic nerve near the bridge of the nose (nasal side) crosses over a place called "opticus" a few centimeters away from the retina to the contralateral hemisphere to continue upwards (below). In the process of continuing to ascend, 10% of the optic nerve reaches the superior nucleus of the midbrain, which is an ancient visual pathway in evolution; and 90% of the optic nerve reaches the lateral knee-like body of the thalamus, where after exchanging neurons, it continues to ascend to the primary visual cortex of the occipital lobe, and there may be a few ascending to other visual cortex layers other than the primary visual cortex, which is a path that has been more recently developed in evolution. As can be seen from the right figure, the information of the right half of the field of vision finally reaches the left hemisphere of the brain after being projected to the left half of the retina of the two eyes, and the information of the left half of the field of vision ends up in the right hemisphere. Therefore, if the right hemisphere is damaged, then some information from the left half of the visual field will not be processed, and the patient will not be able to see. This is why majors and .b. So patients with impaired primary visual cortex in the right hemisphere will not be able to see the left half of the visual field.
The two visual pathways are functionally different. In the old pathway, the superior hill is responsible for the movement of the eye, and the cortical area reached by it upwards is also a brain area related to the action guided by visual cues, which are generally automatic behaviors and do not enter consciousness. New pathways continue to ascend to the higher visual cortex after reaching the primary visual cortex, and the result of these cortical activities either enters consciousness or not (more on that later). However, the functioning of the primary visual cortex is a prerequisite for the subject to be aware of what he sees. Blind patients, because of damage to the primary cortex, visual information from the retina cannot enter the brain area related to consciousness, so the patient reports that he sees nothing; but his old pathway is complete, and the information from the retina can still reach the superior hill, and even further reach the brain region that acts under the guidance of visual cues, so the patient can also perform automatic behavior.
A few years ago, scientists found that there are many nerve cells in the brain that respond to the environment in rats, including "position cells" that detect spatial positions, "head-facing cells" that detect their head orientation, and "boundary cells" that detect obstacles. A similar form of cell may also be present in the human midbrain, one of the possible mechanisms by which the superior mothalamus contributes to blindness.
In addition to old visual pathways, evidence in recent years suggests that the lateral knee-like body may also contribute to blindness. Experiments have shown that there is also a direct link from the lateral knee to the striated epidermis (near the primary visual cortex), and in particular to v5 (an area in the striated epidermis) responsible for motor sensation, although its number is much less than the connection from the lateral knee to the primary visual cortex. Functional magnetic resonance testing found that after the primary visual cortex is completely damaged, there are still some residual activities in the striated exodermal layer, but if the lateral knee is injured, then these residual activities disappear. Therefore, the contribution of the lateral knee to blindness cannot be excluded. The magnitude of the contribution of these possible mechanisms to blindness needs to be further studied.
In any case, the blind patient's eyes must be good so that information from the optic nerve can reach the superior thalamus and lateral knee. If the eye is damaged, then neither the new or the old pathway can receive visual information, and there is no blindness. Ordinary blind people are blind precisely because of eye damage, so they do not show blindness.
A blind person who can [walk freely].
Below is the story of a patient with damage to both primary visual cortexes. Because his entire primary visual cortex was destroyed, his entire field of vision became a blind spot. His story is a more vivid illustration of what miracles a patient who is completely blind and blind can perform.
T. N. is a Burundian doctor who works for the World Health Organization in Switzerland. Unfortunately, in 2003, when he was half a hundred years old, he suffered two consecutive severe strokes in just 5 weeks, the first of which damaged his left visual cortex and the second of which damaged his right visual cortex. Follow-up examination revealed that his entire primary visual cortex had been destroyed, so although his eyes were still intact, he could not see anything at all, at least as he himself said.
Dutch neuroscientist Beatrice de gelder performed routine vision checks on him, and according to these criteria, only the results of these tests can be judged to be completely blind. The results of the MRI showed extensive structural damage to his primary visual cortex, with the superior thiloid being intact. Functional magnetic resonance imaging shows that when shown to him, his primary visual cortex has no activity. That is, his new visual pathway has been completely interrupted at the primary visual cortex, while his old visual pathway remains intact.
In 2008, Degelde confronted T. n. An experiment was done. She led him into a corridor that was cluttered with stuff and told him that the corridor was empty so he could walk through it on his own without using a pathfinder. To be on the safe side, she asked Weisskrontz to follow T. n. Behind you in case of an accident. Result t. He walked smoothly, avoiding all the obstacles, and he even knew to squeeze sideways between the nearby trash can and tripod, though he had no idea why he had to turn sideways. He went around the cardboard box on the ground, but he didn't know why he was taking a detour, and he didn't know what he was hiding, or even if there was anything in front of him. In other words, he didn't have conscious vision, but he subconsciously avoided all obstacles!
Afterwards, he said he was just going as he pleased, not because he knew something was on the way. In addition, when the image of the terrified face was shown to him, he frowned despite saying that he did not see anything, and the MRI indicated that his amygdala was active. The amygdala is an important area of the brain that is related to emotions, especially fear. Because his primary visual cortex was completely damaged and his superior, lateral knee, and amygdala were intact, he was unaware of what he saw, but was able to avoid obstacles and react emotionally to images with strong emotional influences.
Degelde vs. t. N. 's miraculous performance comments: "This makes us realize that even if we are no longer aware of what we see, and we cannot intentionally avoid obstacles, there is still something that man can do. This tells us how important the ancient neural pathways of evolution are, and that they play a much more role in the real world than we think. "This part of our vision is dedicated to moving around the world and moving around, not to recognize things." We're always calling back resources hidden in our brains, doing things we thought we couldn't do on our own. ”
This article is an excerpt from Science World, Issue 9, 2016
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