Yes, blind people do indeed dream in visual images. For people who were born with eyesight and then later went blind, it is not surprising that they experience visual sensations while dreaming. Dreams are drawn from memories that are stored in the brain as well as from brain circuitry that is developed while experiencing the outside world.
Therefore, even though a person who lost his vision may be currently blind, his brain is still able to draw on the visual memories and on the related brain circuits that were formed before he went blind. For this reason, he can dream in visual images. What is more surprising is the discovery that people who were born blind also dream in visual images.
The human experience of vision involves three steps:
(1) the transformation of a pattern of light to electrical impulses in the eyes
(2) the transmission of these electrical impulses from the eyes to the brain along the optic nerves
3) the decoding and assembly of these electrical impulses into visual sensations experienced in the brain
If any one of these three steps is significantly impaired, blindness results. In the vast majority of cases, blindness results from problems in the eyes and in the optic nerves, and not in the brain. In the few cases where blindness results from problems in the brain, the person usually regains some amount of vision due to brain plasticity (i.e. the ability of the brain to rewire itself). Therefore, people who are blind since birth still technically have the ability to experience visual sensations in the brain. They just have nothing sending electrical impulses with visual information to the brain.
In other words, they are still capable of having visual experiences. It's just that these experiences cannot originate from the outside world. Dreams are an interesting area, because dreams do not directly originate from the outside world. Therefore, from a plausibility standpoint, it is possible for people blind since birth to dream in visual images. However, just because blind people have the neural capacity to experience visual sensations does not automatically mean that they actually do. Scientists had to carry out research studies in order to determine if people blind since birth actually do dream in visual images.
At this point, you may be wondering, "Why don't we just ask the people blind since birth if they dream in visual images?"
The problem is that when you ask such people this question, they will always answer no. They are not necessarily answering no because they actually do not have visual dreams. They are saying no because they do not know what visual images are. A girl with eyesight visually recognizes an apple because at some point in the past she saw the apple and ate it, and therefore is able to connect the image of an apple with the taste, small, shape, and touch of an apple. She is also able to connect the image with the word "apple."
In other words, the visual image of an apple becomes a trigger for all the memories and experiences she has previously had with apples. If a girl has never personally experienced the visual image of an actual apple, then seeing an image on an apple in a dream for the first time has no connection to anything in real world. She would not realize she is seeing an apple. As an analogy, suppose you have never tasted salt.
No matter how much people describe salt to you, you do not know what the experience is really like until you experience it personally. Suppose you were all alone and you came across a bag of very salty potato chips in an unlabelled bag. When you eat the chips, you would experience the taste of salt for the first time, but you would not know that that is what you are experiencing, because you would no other previous experiences or connections with it.
Similarly, people blind since birth have no experience of connecting visual sensations with external objects in the real world, or relating them to what sighted people describe as vision. Therefore, asking them about it is not useful.
Instead, scientists have performed brain scans of people blind since birth while they are sleeping. What scientists have found is that these people have the same type of vision-related electrical activity in the brain during sleep as people with normal eyesight. Furthermore, people blind since birth move their eyes while asleep in a way that is coordinated with the vision-related electrical activity in the brain, just like people with normal eyesight.
Therefore, it is highly likely that people blind since birth do indeed experience visual sensations while sleeping. They just don't know how to describe the sensations or even conceptually connect in any way these sensations with what sighted people describe as vision.
With that said, the brain scans during sleep of people blind since birth are not identical to those of sighted people. While people blind since birth do indeed dream in visual images, they do it less often and less intensely than sighted people. Instead, they dream more often and more intensely in sounds, smells, and touch sensations.
We should keep in mind that a person blind since birth has never had the experience of seeing images originating from the external world and therefore has never formed visual memories connected to the external world. The visual components of their dreams therefore cannot be formed from visual memories or the associated circuitry. Rather, the visual sensations must arise from the electrical fluctuations that originate within the brain.
What this means is that people blind since birth probably do not experience detailed visual images of actual objects such as apples or chairs while dreaming. Rather, they probably see spots or blobs of color floating around or flashing. The spots may even correlate meaningfully to the other senses. For instance, a dream of a police car siren sound traveling from the left to the right may be accompanied by the visual sensation of a spot of color traveling from the left to the right at the same speed.
In summary, the current evidence suggests that people blind since birth do indeed dream in images, but we do not know exactly what they see.
On a related note, brain scans have found that all humans dream in visual images before they are born. The darkness of the womb leads to the fact that none of us experienced actual vision before we were born. Therefore a fetus is equivalent in experience to an adult that was blind since birth. For this reason, it should not be surprising that fetuses also dream in visual images.
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