Strange Visual Phenomenon
February 27, 2014 2:25 AM   Subscribe

Twice in the last few months I've been in the car at night and experienced a strange visual phenomenon. I know YANMD, I'm sure its nothing but I'm wondering if it has a name or if it could mean something worse.

What happens is that when looking at the little red man (not necessarily looking at it, but able to see it) and turning my head the image of the little red man repeats like a chain of paper dolls. Each man is as bright as the real one and the image stays for at least a second, maybe 2. While sitting at the lights I was able to replicate it several times in both directions on the same light, nothing else in the scene was duplicated that I noticed. I was able to do it again a few minutes later on a different red man but it didn't happen every time I moved my head or with any other lights. It happened before a few months ago but I can't remember whether it was just the red man last time or whether it was also the green man or other lights too. It did happen several times during the journey though (fwiw, I wasn't driving in either case)

Does this phenomenon have a name? Is it normal?
posted by missmagenta to Health & Fitness (12 answers total) 2 users marked this as a favorite
Red LEDs cause me some similar problems. I have no idea if this is a 'thing'.
posted by efalk at 2:44 AM on February 27, 2014

Strictly anecdotal, but years ago I noticed a similar phenomenon with not only the little red man, but other objects out my car window at night: streetlights, whatever color was illuminated on the traffic light etc. Always when looking to the side with my eyes (not by turning my head), or out the corner of my eye. After a while I started to notice it during the day, as well; when it started causing me difficulty in reading documents that I was trying to type (which were always situated to the far right or left of my typewriter) I mentioned it to my eye doctor during my next check-up. Turned out I had some sort of muscle problem in my eyes, and I had to get prisms in my eyeglass prescription.
posted by Oriole Adams at 2:44 AM on February 27, 2014

The persistence is perhaps a little longer than I generally get, but the trail of paper dolls thing is probably because the red man you were looking at was made out of LEDs rather than being the older-style red filter in front of an incandescent bulb.

The human eye is sensitive to peak brightness, so high power LEDs are usually driven by a pulsed electricity supply that makes them very bright during current pulses but actually leaves them turned off for more than half the time; that way, you get much more apparent brightness out of them than you would by running them steadily at the same average power.

The pulse repetition rate is high enough that LEDs driven that way don't generally flicker visibly, but if they move across your visual field they make a trail of discrete images rather than the smeared out trail you'd get from a continuous light source.
posted by flabdablet at 2:59 AM on February 27, 2014 [11 favorites]

Fascinating! Is there a reason why one person would be able to see it and someone else wouldn't?
posted by missmagenta at 3:19 AM on February 27, 2014

Many people just don't pay attention to phenomena like these, or if they do notice them, don't bother to wonder about them.
posted by flabdablet at 4:42 AM on February 27, 2014 [3 favorites]

Oh, one more thing: your retina has a mixture of rod cells that respond well to low intensities of light but are quite slow, and three different kinds of cone cell that respond to different wavelengths (forming the basis for color vision) and are quite fast but need more light to trigger at all.

Your brain integrates information from all of these: having identified a particular object as red, your brain will continue to perceive it as red even if it's not still getting a cone-based stimulus for it any more. So it's highly likely that you're getting a sequence of paper-doll red men as the flickering LEDs trigger your cones, and that the persistence is due to the slowness of your rods, which register where the light was without contributing color information.

There are many more cones in the center of your retina than near the edges, so stuff you catch out of the corner of your eye can be expected to persist for longer.

You can observe a related visual phenomenon by sitting outside in starlight, only just bright enough to see nearby stuff around you, looking at a distant well-lit object, and then moving your head. You'll notice that the bright object seems to slide around relative to everything else. That's because most of the visual field in your low-light surroundings is defined by slow rods, which makes your brain use that as the frame of reference, while the information you're getting about the well-lit thing is coming from your much faster cones.

The brain really does do a hell of a lot of fill-in and interpolation. Our visual field is really nowhere near as detailed as we perceive it to be - outside the very center it's actually quite vague, but the whole things seems to be in glorious hi-def because that's what everything we actually look at looks like. This is the entire basis for sleight of hand: if you can get your audience not to look at a hand you're using for a trick, you can whip it in and out of the outer parts of their visual fields without them registering that at all, while they remain locked into the blissful illusion that what they're seeing is all there is.

If you don't believe that, try reading text that's actually 30° off to the side of where you're looking. You won't even be able to make out the letter shapes.
posted by flabdablet at 5:03 AM on February 27, 2014 [5 favorites]

I have a condition called esophoria which, in the days before I had a diagnosis, would lead to my eyes becoming fatigued and holding a fixed double image for distant images. The test for this, which I remember 20 years later, was showing me a red LED in a darkened room, which my eyes were unable to resolve as a single image. Apparently there is something about the color red that is difficult to bring together for people with inward convergence deficiencies. Anyway, I think the color is probably a key detail in this, and then the brain fill-in effects probably explain the rest.
posted by norm at 5:44 AM on February 27, 2014 [1 favorite]

norm that's awesome to know, thank you for sharing. We have a family history of convergence insufficiency (instead of divergence insufficiency) and I've noticed issues with red LEDs in newer cars and in stoplights (though I'd think the windscreen would be enough of a filter ...). It's something I'll warn my future drivers on in a few years ...
posted by tilde at 5:55 AM on February 27, 2014

For what it's worth: Seattle has dynamic speed limits, with LED signs on some freeways. I often get multiple images from those even in broad daylight.
posted by wotsac at 8:33 AM on February 27, 2014

Many people just don't pay attention to phenomena like these

There's more to it than this. My wife literally can't see this phenomenon, and I'm apparently extraordinarily highly attuned to (and annoyed by) it, as we discovered during an argument about why I was throwing away perfectly good christmas tree lights (because the strobe effect every time I moved my eyes was driving me insane).

We did Sciencetm and she just doesn't see the strobe effect at all, even when she knows what she's looking for. I can't not see it and find it highly distracting -- it's getting to the point that I have trouble driving at night because so many cars have LED taillights now.
posted by ook at 10:27 AM on February 27, 2014

My wife literally can't see this phenomenon

I suspect the same may have been true of the engineer who originally picked the customary strobe rate for these things.
posted by flabdablet at 3:17 PM on February 27, 2014 [1 favorite]

Your eyes basically become useless when they're moving. The smears across your retina don't contain any useful information, and so your brain doesn't even bother passing them through to your brain. This is called Saccadic Masking. However, it's not a complete suppression, and sometimes information leaks through, as in your case.

I have an interesting visual anomaly in that saccades, for me, are somewhat voluntary, much like breathing. I can choose at any time to have a saccadic mask or not, much as you can choose to breathe or not, but if you don't think about it, your body picks up the slack. Because of this, I can flick my eyes around and see smears, while high-frequency flickering lights resolve into a line of dots or individual images. I've never had a chance to test it properly, but I believe that - with the proper setup - I could tell the difference between a solid light and a 2 kHz flashing light. I have been able to see blinking lights like this since I was a child. I remember telling my dad that the LED on his CB was blinking and him not believing me when I was about 7-8 years old.

The same thing is happening to you. Your visual cortex, for whatever reason, didn't completely shut off your eyes during that movement, so the brief moments that the stick man was lit up resulted in a string of images, each one corresponding to a single flicker.

If you'd like to study the effect, your camera phone probably does the same thing. I can whip mine around in a scene, and see flashing lights stand out of the blurs that the rest of the scene turns into; I use this to show my friends what I see when I move my eyes around.

On a side note, I hate Hate HATE the LED taillights that flash at 100 Hz or so.
posted by Hatashran at 8:44 PM on February 27, 2014

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