My vision is normal. When I close my eyes or am in a dark environment, I feel as though I am looking at blackness, or have a black field in front of my eyes. Subjectively, it seems to take up about 180 degrees in front of me, although I know it’s not that wide or tall. On the other hand, I don’t have any expected perception of things off to my sides, or behind my head: I don’t ‘see’ blackness behind me. I have read that persons with early visual loss have no sense of ‘blackness’, or an expected visual field, which makes sense to me. But what about people with blindness acquired later in life - let’s assume due to eye/retina/optic nerve causes rather than cortical blindness. Do they continue to ‘perceive’ blackness in front of them, or does that perception go away with time and lack of input to the visual centers of the brain?
I guess this is partly a question about the plasticity of the visual cortex, too. Without input, is the visual cortex rewired for other purposes?
It’s all over the place depending on the type of blindness, the person, etc.
But there are remarkable things in some people. E.g., some people who lose half their visual field do not see a blank half field or any such. Some even deny that they are not seeing a lot of what is right in front of them. The brain just works extra hard to patch over things so the person doesn’t notice a thing wrong with their vision.
They are so many layers between the basic visual detection to “Hey, there’s a rabbit over there!” that it’s possible to understand how people just don’t notice the missing parts of their field.
A small scale, everyday, example of this are the blind spots in each of your eyes. The standard test where you look at a dot with one eye and a “+” near it disappears shows what the brain does. There isn’t a “black” spot or anything where the “+” is, the background is the same color as the rest of the paper. (You can try this with different colored papers.)
Just think of this filling in as being done for a lot more of the visual field.
I lost all of the vision in my left eye in 2004 or so. I don’t have the sensation of having a huge blank in my visual field; the stuff on the left is just like the stuff behind my head, invisible.
One interesting thing is that when I leave a well lit environment going into a dark room, like when I go into my bedroom which is lit only by a couple of dim lights on electronic devices, there’s a brief period where I get the sensation that the little bit of sight is coming in through the dead eye. Once the good eye adapts it goes away. In my head I call it blindsight.
I know it’s not coming in through the left eye, I can literally close my good eye and stare at the sun. Nothing comes through, much less the power light on my Roku.
This isn’t quite what you’re asking, but I hope it’s relevant. I have retinal damage and thus have severely limited peripheral vision. Before the retinal damage, I perceived my field of vision as you do. Now, I…perceive my field of vision as you do. The difference is that while I perceive it that way, in actuality, it isn’t there. It’s not black or grey or blank off to the sides. Yet if you wave your hand at the 140 degree mark, it simply doesn’t exist for me. And in the Goldman Field Vision test, I can’t see a light out there, even though I seem to perceive lights and colors while walking around (and running into people, street signs, etc. that are beyond my limited visual field. I can only conclude that my brain is trying to fill in the blank.
My retinas are fine, but I have had migraine aura that involved visual field loss (homonymous hemianopia), where I was unaware of any defect in my vision until I realized I couldn’t read road signs. Yes, I was driving at the time. Yes, I pulled over ASAP. My recollection was not of seeing half a blank ‘screen’, but that the field of view was normal sized even though it wasn’t. That’s part of the reason for my question: this was a central process involving the visual cortex, and I was wondering about whether the same thing happened in cases where the visual cortex was normal but something happened to the eyes.
If you haven’t read the book “The Mind’s Eye” by Oliver Sacks, you may find it to be of interest. It descibes cases involving various disturbances to the visual system, including one he experienced.
You may also find his book “Migraines” interesting.
From Wikipedia:
Even though the blind are no longer able to see, the visual cortex is still in active use, although it deals with information different from visual input. Studies found that the volume of white matter (myelinated nerve connections) was reduced in the optic tract, but not in the primary visual cortex itself. However, grey matter volume was reduced by up to 25% in the primary visual cortex. The atrophy of grey matter, the neuron bodies, is likely due to its association with the optic tract.[2] Because the eyes no longer receive visual information, the disuse of the connected optic tract causes a loss of grey matter volume in the primary visual cortex. White matter is thought to atrophy in the same way, although the primary visual cortex is less affected.
For example, blind individuals show enhanced perceptual and attentional sensitivity for identification of different auditory stimuli, including speech sounds. The spatial detection of sound can be interrupted in the early blind by inducing a virtual lesion in the visual cortex using transcranial magnetic stimulation.[3]
The somatosensory cortex is also able to recruit the visual cortex to assist with tactile sensation. Cross modal plasticity reworks the network structure of the brain, leading to increased connections between the somatosensory and visual cortices.[4] Furthermore, the somatosensory cortex acts as a hub region of nerve connections in the brain for the early blind but not for the sighted.[5] With this cross-modal networking the early blind are able to react to tactile stimuli with greater speed and accuracy, as they have more neural pathways to work with. One element of the visual system that the somatosensory cortex is able to recruit is the dorsal-visual stream. The dorsal stream is used by the sighted to identify spatial information visually, but the early blind use it during tactile sensation of 3D objects.[6] However, both sighted and blind participants used the dorsal stream to process spatial information, suggesting that cross modal plasticity in the blind re-routed the dorsal visual stream to work with the sense of touch rather than changing the overall function of the stream.
Tangent but interesting:
There is a light reception system identified in the last 10 or 15 years that uses a different mechanism than rods and cones and primarily forwards the signal to mood and circadian rhythm processing areas. But, if I remember correctly, some of this signal does flow to the visual system. Scientists think this may be the mechanism behind some blind people’s apparent ability to detect some visual information without being aware of the source.
I used to play tennis with a friend, and then she lost the vision in one eye and had to quit because she couldn’t manage to hit the ball. No depth perception, everything looked flat.
After three years, suddenly she realized she had adapted and now had depth perception again. She didn’t know when it happened, but it happened and she said it now looked the same to her as it had before, with two eyes.
Brains are amazing.
Since the question has been sort of answered, I’ll go on a small tangent.
When I was in junior high, I hit my head and sustained a concussion. At the time of the injury, I had actually been knocked unconscious. While there is a lot that I don’t remember about that day, I do remember being unconscious. It turns out that consciousness isn’t a simple switch of either you are or you aren’t. I was only mostly unconscious. I couldn’t move or speak but I was aware that I was laying down. I also knew who was standing around me, even after they moved, which was weird because I was sure I couldn’t see. It was likely that my eyes were open but I wasn’t conscious enough to process all the visual information.
