Tag Archives: Blind spot

The brain is far more wired up than we thought

The hardest thing in experimental psychology is being smart enough to think of something truly clever.  Here is a beauty, showing that the brain is far more wired together than we ever thought.

First some background.  You’ve probably heard of the blind spot (although you’ve never seen it).  It’s the part in your eye were all the never fibers from the sensory part of the eye (the retina) are collected together forming the optic nerve.  Through an ophthalmoscope it appears like a white oval (largest diameter under 2 milliMeters)  It’s white because it’s all nerve fibers (1,000,000 of them) with no sensory retina overlying it.  So if you shine a very narrow light on it, you’ll see nothing.   That’s the blind spot.

Have a look at https://en.wikipedia.org/wiki/Visual_system. Both eyes project to both halves of he brain.  Because the blind spot is off to one side in your visual field, the other eye maps a different part of the retina to the same area of the brain.  But if you patch that eye, on one side of the brain the blind spot gets no input.

 

 In the healthy visual system, the cortical representation of the blind spot (BS) of the right eye receives information from the left eye only (and vice versa). Therefore, if the left eye is patched, the cortex corresponding to the BS of the right eye is deprived of its normal bottom-up input.

Proc. Natl. Acad. Sci. vol. 117 pp. 11059 – 11067 ’20 https://www.pnas.org/content/pnas/117/20/11059.full.pdf

Hopefully you’ll be able to follow the link and look at figure 1 p. 11060 which will explain things.

Patching the left eye deprives that area of visual cortex of any input at all.

Here comes the brunt of the paper — within minutes of patching the left eye, the cortical representation of that spot begins to widen.  It starts responding to stimuli from areas outside its usual receptive field.

Nerves just don’t grow that fast, so the connections have to have been there to begin with.   So the brain is more wired together than we thought.  Perhaps this is just true of the visual system.

If not, the work has profound implications for neurologic rehabilitation.

I do apologize for not being able to explain this better, but the work is sufficiently important that you should know about it.

Addendum 4 June — here’s another shot at explaining things.

    • As you look straight ahead, light falls on the part of your retina with the highest spatial resolution (the macula). The blind spot due to the optic nerve is found closer to your nose, which means that in the right eye, the retina surrounding the blind spot ‘sees’ light coming from toward your ear. Light from the same direction ( your right ear) will NOT fall on the optic nerve of your left eye (which is toward your nose) so information from that area gets back to the brain (which is why you don’t see your blind spot).

      Now visual space (say looking toward the right) is sent back to the brain coherently, so that areas of visual space transmitted by either eye go to the same place in the brain.

      So if you now cover your left eye, there is an area of the brain (corresponding to the blind spot of the right eye) which is getting no information from the retina at all. So it is effectively blind. Technology permits us to actively stimulate the retina anywhere we want.. We also have ways to measure activity of the brain in any small area (functional MRI). Activity increases with visual input.

      Now with the left eye patched, stimulate with light directed at the right eye’s blind spot. Nothing happens (no increase in activity) in the cortical area representing that part of the visual field. It isn’t getting any input. So it is possible to accurately map the representation of the right eye’s blind spot in the brain in terms of the brain areas responding to it.

      Next visually stimulate the right eye with light hitting the retina adjacent to the right eye’s blind spot. Initially the blind spot area of the brain shows no activity, After just a few minutes, the area of the brain for the right eye’s blind spot begins to respond to stimuli it never responded to initially. This implies that those two areas of the brain have connections between them, that were always there, as new nerve processes just don’t grow that fast.

      To be clever enough to think of a way to show this is truly brilliant. Bravo to the authors.