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Eyesight Question

  1. Jul 4, 2013 #1


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    When you look at a very bright source of light, such as the Sun, what causes the pain reflex and makes you close your eyes? I ask because I've read that IR light, typically from lasers, will not trigger this response, so I assume that the reflex isn't caused by your eye being damaged, but by something else.

    Also, if someone were to be blind due to a defect in their brain, and their eyes were completely functional, would they experience pain when they looked at a bright source of light?
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  3. Jul 4, 2013 #2
    I'm not sure, a quick search turned up essentially nothing.

    Depends on which part of the brain. If you're talking about the visual cortex, my guess is that a defect or damage there would not affect the squint reflex or the experience of pain, because these are most likely mediated by the cranial nerves at the level of the brainstem.
  4. Jul 4, 2013 #3
    You don't actually feel pain when this happens, as the rear of the eye contains relatively few pain neurons. The pain felt is a psychosomatic response. What you sense as pain from a bright flash is the flood of signals sent from over excited rods and cones in one big rush of information delivered via the optic nerve. The overwhelmed visual cortex mistranslates this input rush as pain. The same reason is also why after a bright light is viewed, you will continue to see ghost images of the flash even with your eyes closed (over excited rods & cones still transmitting after the stimulus has been removed).

    As an aside, this phenomenon is also why loud noises are painful and your ears ring afterwards. Same psychosomatic response mechanism.

    Your rods and cones are optimized for visible light as they are stimulated by it. They are insensitive to other wavelengths, so this will not happen (even if the source is to be so powerful as damaging). You don't feel your eyes being damaged by UV light for the same reason. The real pain comes afterwards once the injured tissue becomes inflamed. Many a newbie metal welder learn this the hard way by thinking he/she can peek at their ongoing welding without the annoying dark glass protection blocking most of what can be seen when the welding arc is not struck.

    <edit> To add, you can achieve a similar psychosomatic response with your sense of touch. Place your hand in very warm water for a few. Then stick it in ice water. It will hurt. The dramatic signal shift from very warm to very cold will trigger a similar overload in your cortex even though the very warm or cold water are inherently not painful alone.
    Last edited: Jul 4, 2013
  5. Jul 4, 2013 #4
    Do you have valid references for these "psychosomatic" responses/mechanisms and "cortical overload"?
    Last edited: Jul 4, 2013
  6. Jul 4, 2013 #5
    I quick looked on line for a convenient link and found a dearth of material on the topic in general. There's a bunch of material about how autistic people have a much lower threshold concerning this across their sensory spectrum as a pathology of this response mechanism though.

    My principle source is this:

    Human Anatomy and Physiology, 8th Ed.
    ISBN 978-0-8053-9569-3

    The topic was covered extensively during several related courses too. While the pain felt is very real (hey, you felt it), the route is not via the innerved pathways that transmits pain and related sensations (such as when you step on a nail in a board or strain your back, etc). This is why the OP is correct in assessing that bright IR light will not cause pain, but bright visible light will. The sensors (rods & cones) cannot transmit the overwhelming signal strength since they do not respond to that light spectrum.

    I guess there is a similar pathway for smell too, but I have never heard of it. It definitely exists for sight, sound and touch (where pain separately exists as a sensory element too).
  7. Jul 4, 2013 #6
    There are specific receptors and neural pathways for pain and I'm not sure "cortical overload" is a scientifically meaningful concept and why, in any case, it should be perceived as pain. In these forums you are required to provide links to sources that can be viewed except in cases where it is generally accepted and well known science. Any pain with your example of plunging a finger into cold and then warm water is likely to be the result of stimulating vascular pain receptors as a result of the sudden narrowing followed by dilatation of blood vessels (vasomotor instability). There is also referred pain which is conveyed by the usual mechanism, but is interpreted at the cortical level as being in a different location than the true source of the stimulus. I'm not saying that psychosomatic pain does not exist, but you must provide evidence that it is relevant in this case. It's too easy to dismiss an undiagnosed pain source as "psychosomatic".
    Last edited: Jul 4, 2013
  8. Jul 4, 2013 #7
    I agree with Vande on this point. The term and concept of "psychosomatic" doesn't hold a lot of water at PF unless perhaps it is well researched and referenced and given to one specific instance. I think you (Leeboy) are right by saying that the pain and the wince involved with a sudden blast of light is due to a sudden assault on the rods and cones of light in the visible spectrum, but I think your assessment that it is a psychosomatic result at the level of the visual cortex is misplaced. The sudden assault most likely triggers nociceptive neurons in the retina and/or related subcortical cranial nerve pathways. The reason IR radiation does not trigger this pain response suggests that it is indeed the rush of activity on the rods and cones that is responsible.

    However, I have no (ready) evidence to support that hypothesis, which is why I said I'm not sure in my initial post. That's how we roll at PF :tongue:
  9. Jul 4, 2013 #8


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    Oh my, I've stirred the hornets.
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