Bright Idea

I’m sure we have all, at one time or another, had the experience of being dazzled by a bright light. The other day it happened to me, and I noticed something quite surprising about it.

I went outside on a sunny morning after our recent blizzard. The snow was deep and white all around, and the effect was blinding. I fumbled in my pocket for my sunglasses, squinting against the glare.

I’d never really thought about this phenomenon in any detail before, but if I had, I’d have assumed that when we are dazzled by intense light the problem is right at the front end – in our eyes themselves. Like all transducers, they are designed to operate in a certain range, and although that range is very large – in the right conditions the human eye can detect single photons – there are going to be some conditions where the incoming energy is just overwhelming, and at such moments the eye is reflexively closed or averted in order to prevent damage. But in this case I happened to noticed that although the glare was intolerable if I had both eyes open, it was much more bearable if I opened only one. Once I had noticed this, I put it carefully to the test, and the results were consistent.

So what does this mean? There is no way, presumably, that the light exposure (and therefore potential damage) to one eye is affected by whether the other is open or closed; what must be happening, then, is that the system is being overwhelmed somewhere further down the visual pathway, after the signals are combined. The sensation, of course, is of the eyes themselves being overloaded, but as we know, such things can be deceiving. It occurred to me, of course, that perhaps we are simply trying to minimize the discomfort, as we do when we hop from one foot to the other on a hot surface. But it seemed that the glare was quite manageable with one eye, while with both open I had the sensation of being bilaterally dazzled.

So is the brain, then, reacting to protect the eyes? I’d imagine so. It’s difficult to imagine that the brain is protecting itself, because presumably there is no risk of damage from the transduced neural representation of the light. Of course, that might not necessarily be true; the way in which the signal is represented could matter. For example, a high-amplitude audio signal passing through an analog amplifier can damage the circuitry, whereas a digital representation of the same waveform is just a different set of numbers. Nevertheless, I think it is safe to say that what is at risk when we stare at the Sun is not our visual cortex, but our retinas. But if the brain, then, is deciding whether to shield the eyes based only on the cumulative input from both eyes, then it’s missing some potentially dangerous cases. An animal that was already blind in one eye, for example, would be at greater risk for losing sight in the other. I suppose that means that we should assume that a creature in that situation, or one whose eyes fail to converge on the same light source, is already at such an adaptive disadvantage that the additional selection pressure to manage light overload independently for each eye (fancier features cost more) was negligible.

I’ve never seen any mention of this anywhere. Were any of you aware of it? Try it for yourselves.

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