A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why do I see sparks when I cough with my eyes closed?

October 21, 1977

Dear Cecil:

I've gone through life wondering about a childhood discovery that I must resolve before I slip into chronic adult paranoia. Lying in bed with a winter cold, I noticed that when I coughed, I sparked--but only when my eyes were closed. What does this mean? Am I all right? Need we be concerned about the power drain? It would be a shame to short out before my time. Should I avoid coughing in the tub? In the rain?

Cecil replies:

Don't worry, goofball, you're not going to electrocute yourself. The sparks are an optical illusion.

In the first stage of a cough, pressure is built up in the lungs, and for a split second that pressure inhibits the flow of blood through them. This causes a momentary imbalance in the circulatory system, forcing more blood to the head, and, naturally, the eyes.

The eyeball is entwined in a network of blood vessels--you've seen the Visine commercials--that we normally see right through. But when these vessels are slightly overloaded, thanks to the pressure in the lungs, they become a little harder to get around.

At the moment of the cough, when the lung pressure is released, a final wave of pressure travels to the head, and the combined effect of the bloated vessels and this final burst creates enough pressure to stimulate the photoreceptor cells. The sparks you see are the outline of the veins. Usually, the sparks are concentrated at the periphery of the eye, where the network of blood vessels is densest.

Interestingly, children spark more than adults--as you get older, you get bigger and less susceptible to subtle changes in pressure. But you can recapture the bliss of childhood by closing your eyes, looking all the way to the right, and touching the left side of your eyelid. Sparks are less likely than a dull glow, but the principle is the same: pressure on the eye creates an illusion of light.

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