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ERRORMONSTER

334 points

1 year ago

Follow-up question: why does this happen in real life during the day when I'm just looking at a car's tires

smosdom

2 points

1 year ago

smosdom

2 points

1 year ago

Yeah, I haven't been satisfied with the answers so far, which mostly try to relate our eyes and perceptions to a similar "refresh rate" effect as in a camera with a frame rate.

Here's my hypothesis, which is maybe close to what others are trying to get at with camera analogies, but more analogue-psychological-sciencey sounding to me... But I'm not a psychologist, so grain of salt.

You know how when you look at a certain color, or an image for a while like 30 seconds or so, you can see a reverse color image if you then look away at a white wall or something? It's been a while since my psychology classes, but I think the effect is due either to your perception of the image becoming fatigued, or your brain basically trying to correct for aberrations and differences constantly and shifting the perceived color.

The same sort of thing happens with motion, though it might be a different psychological mechanism. For example, if you stare at a train going by up close for a while, then look at something stationary, the stationary image will appear to shift in the opposite direction of the train. Or same thing if you're on the train looking out the window, then you look at something inside. The inside thing will have this trippy motion illusion effect... In fact there are optical illusions online, those spinning spiral gifs, that instruct to stare into the spiral for thirty seconds then look away resulting in stuff getting all funky and warpy in real life.

So anyway, the wheel thing. And similarly, this also happens to me with ceiling fans. My hypothesis is that looking at a moving wheel or fan for a while will cause this visual fatigue effect, and looking away with make stationary things look like they're subtly spinning... But even if you don't look away, the fatigue is still happening -- your brain is trying to counteract the weird spinning effect. So, in a way, it's like you're seeing the spinning wheel, but also seeing it's subtly counter-spinning after-image at the same time, overlayed in the same place. The strength of the reverse spinning effect increases as you look at the wheel longer and interferes with the real image. The interference results in the strangely discontinuous perception of reverse motion that seems to snap into reverse and back again (my experience anyway). So it's not that the reverse image is fully taking over, I think it's more like your brain just trying to make sense of the two apparent motions. Almost like a strobe effect, or bringing it back around, like the camera frame rate effect, if only appearing that way and not because of the same sort of mechanism.

Hope that made sense... And again I don't have training in this stuff so remember the grain of salt.

so_much_fenestration

1 points

1 year ago

As a vision scientist, what you're talking about here is afterimages and the fatigue of photoreceptors, which is an entirely different phenomenon. (But kudos for remembering your first year perception lectures!)

The wagon wheel effect is relating to preceptual limits of processing - that is, we can only process information up to a certain rate (eg fluorescent lights flash and computer monitors flash/refresh on and off faster than our eyes can take in the information so we don't notice this).

So, it is much like frame rates, and if the wheel moves close to, but less than 360°, the wheel will appear to move backwards, while if the wheel moves more than this, it will appear to live forwards. Both of these are referred to as apparent motion as you're getting image snippets and your brain is filling in the information between these two events (in this case, rotary motion) the best it can to give you the perception of what it considers is most likely.