Why do fast-turning car wheels appear to turn slowly?

clop · #1 (**permalink**) 25-August-2007, 11:29 AM

Hello,

I'm sure we've all seen cowboy films where the wagon wheels appear to stop, or be turning slowly backwards or forwards, because the time period between each successive movie frame is equal to, or just shorter or longer than the time it takes each wagon wheel spoke to move to the position occupied by the neighbouring spoke.

But this also happens to me when looking at car wheels in real life. If I'm driving fast on the freeway, and the wheels on the car next to me are rotating rapidly, they can appear to be turning forwards very slowly. It feels like it's happening for the same reason as the wagon wheels.

Does this mean that my eyes are operating in some kind of frame mode? Surely this would mean that the output of my foveal (and retinal?) cells would have to be synchronised in some way?

If our eyes are working in frame mode, what is the frame rate? Can the frame rate change and why? Does an individual's frame rate correlate with anything?

clop

01101001 · #2 (**permalink**) 25-August-2007, 01:29 PM

Quote:

Originally Posted by clop

But this also happens to me when looking at car wheels in real life.

Under what lighting conditions?

Wikipedia: Wagon-wheel effect

BigDon · #3 (**permalink**) 25-August-2007, 11:11 PM

Nice reply 105.

clop · #4 (**permalink**) 26-August-2007, 12:02 AM

Quote:

Originally Posted by 01101001

Under what lighting conditions?

Wikipedia: Wagon-wheel effect

Yes, excellent reply, thanks.

Quote:

Many people report seeing the wagon-wheel effect on car wheels under continuous illumination. The effect can be produced with a special pattern of lug nut orientation, but often there are other explanations. Some cars have special wheels called spinners; these can truly rotate backwards. With conventional wheels, there's always the possibility of stroboscopic illumination. At night, it can come from artificial light sources. During the day, it can come from reflections from another car's wheels that are rotating at a slightly different rate from that of the observed wheel, or even from another wheel of the observed car if its diameter is not exactly the same as that of the observed wheel. The same caution needs to be exercised for propellors if other propellors are spinning nearby.

Rushton (1967[3]) observed the wagon-wheel effect under continuous illumination while humming. The humming vibrates the eyes in their sockets, effectively creating stroboscopic conditions within the eye. By humming at a frequency of a multiple of the rotation frequency, he was able to stop the rotation. By humming at slightly higher and lower frequencies, he was able to make the rotation reverse slowly and to make the rotation go slowly in the direction of rotation. A similar stroboscopic effect is now commonly observed by people eating crunchy foods, such as carrots, while watching TV. The crunching vibrates the eyes at a multiple of the frame rate of the TV. Besides vibrations of the eyes, the effect can be produced by observing wheels via a vibrating mirror. Rear-view mirrors in vibrating cars can produce the effect.

The first to observe the wagon-wheel effect under truly continuous illumination (such as from the sun) was Schouten (1967[4]). He distinguished three forms of subjective stroboscopy which he called alpha, beta, and gamma: Alpha stroboscopy occurs at 8-12 cycles per second; the wheel appears to become stationary, although "some sectors [spokes] look as though they are performing a hurdle race over the standing ones" (p. 48). Beta stroboscopy occurs at 30-35 cycles per second: "The distinctness of the pattern has all but disappeared. At times a definite counterrotation is seen of a grayish striped pattern" (pp. 48-49). Gamma stroboscopy occurs at 40-100 cycles per second: "The disk appears almost uniform except that at all sector frequencies a standing grayish pattern is seen ... in a quivery sort of standstill" (pp. 49-50). Schouten interpreted beta stroboscopy, reversed rotation, as consistent with there being Reichardt detectors in the human visual system for encoding motion. Because the spoked wheel patterns he used (radial gratings) are regular, they can strongly stimulate detectors for the true rotation, but also weakly stimulate detectors for the reverse rotation.

Purves, Paydarfar, and Andrews (1996[5]) also reported reversed rotation of radial gratings. They concluded however, that this was evidence that human visual perception takes a series of still frames of the visual scene and that movement is perceived much like a movie. This can be called the discrete-frame theory.

Seems it might not be as simple as a stroboscopic light source though. Vibration of the eyes in the eye sockets?

clop

novaderrik · #5 (**permalink**) 26-August-2007, 02:45 AM

could have something to do with the way your eyes are always moving and scanning everything in the field of view.
or it could be your brain interpreting things in "bullet time" like in movies when they slow down a speeding bullet.

jami cat · #6 (**permalink**) 26-August-2007, 04:08 PM

It happens under Artificial lighting, which is at a 60hz flicker.

I don't recall seeing this in natural sunlight.

But, I'll be on the lookout.

I like the freaky chain-link-fence effect too...

captain swoop · #7 (**permalink**) 27-August-2007, 12:38 AM

wagon Wheels used in movies have spokes that have uneven spacing to stop the 'turning backwards' effect.

jami cat · #8 (**permalink**) 27-August-2007, 09:54 PM

Quote:

Originally Posted by captain swoop

wagon Wheels used in movies have spokes that have uneven spacing to stop the 'turning backwards' effect.

Uneven spacing or using an Odd number like 11 that isn't divisable by the frames per sec?

I noticed that wagon wheels tend to be even numbered...after Googling.

Hmm...http://future.iftf.org/2004/08/wagon_wheel_ill.html

Quote:

"Purves goes on to suggest that this effect demonstrates how our perception of time is carved into packets of interaction with our neural systems. Marketing implications?"

Hmm2...http://zone.ni.com/devzone/cda/tut/p/id/3000

Quote:

The Antialiasing button in video card settings?

clop · #9 (**permalink**) 27-August-2007, 11:17 PM

Quote:

Originally Posted by jami cat

Uneven spacing or using an Odd number like 11 that isn't divisable by the frames per sec?

I don't think using an odd nuimber of spokes would make any difference. It doesn't matter whether the frame rate divides by the number of spokes. Whether you see the effect or not just depends on whether the time period between frames is similar to the time period it takes a spoke to move to another spoke.

clop

hhEb09'1 · #10 (**permalink**) 29-August-2007, 10:33 PM

Quote:

Originally Posted by clop

Quote:

The effect can be produced with a special pattern of lug nut orientation,

The pattern need not be that special, this page points out that the lug nuts might end up all aligned, because of the use of the tools to tighten them:

Quote:

So, what happens if we align all the nuts parallel to a single line? This would happen if the nuts were tightened with a tire iron, and the angle of the handle was the same each time. This pattern gives annoying flashes, but it also gives a backwards-drifting pattern which is 6x slower than the wheel RPM

MentalAvenger · #11 (**permalink**) 31-August-2007, 03:24 AM

In one of my nighttime college psychology classes, the instructor was showing us the standard rotating discs. Changing the speed made the pattern stand still or change speed forwards or backwards. Different patterns produced different results, including blue and pink lines from black and white patterns on the wheels. She explained that the effect was caused by the human eye which took “pictures” at a certain rate.

I objected. I explained that the effect was caused by the strobing of the lights in the classroom. In response, she assigned me the project of proving my point. I’m sure she thought I would fail. The next night I brought an incandescent bulb lamp, and a battery powered lantern. I went through the set of wheels with the fluorescent room lights, and the results were the same as the night before. Then I shut off the room lights and ran the test with the incandescent lamp. The results were similar, with the strobe effect making patterns stand still, slow down, and reverse etc. But the blue and pink effect went away, as I knew it would, being an effect of the spectrum of the fluorescent light. Then I ran the tests with only the battery operated lantern. At no time did anyone in the class see anything but a blur from any of the wheels, regardless of speed. The instructor didn’t say anything.

The instructor didn’t like me much after that.

Ilya · #12 (**permalink**) 06-September-2007, 07:20 PM

Quote:

Originally Posted by clop

Does this mean that my eyes are operating in some kind of frame mode? Surely this would mean that the output of my foveal (and retinal?) cells would have to be synchronised in some way?

If our eyes are working in frame mode, what is the frame rate? Can the frame rate change and why? Does an individual's frame rate correlate with anything?

Your eyes DO work in frame mode: http://en.wikipedia.org/wiki/Saccade

But that's not what causes wagon-wheel effect. (I suspect MentalAvenger's professor was claiming otherwise.)

MentalAvenger · #13 (**permalink**) 08-September-2007, 07:24 AM

Your Wikipedia article has nothing to do with “frame rate”, or anything associated with it.

hhEb09'1 · #14 (**permalink**) 12-September-2007, 05:45 AM

Quote:

Originally Posted by MentalAvenger

Your Wikipedia article has nothing to do with “frame rate”, or anything associated with it.

From the link: "In addition to the kind of saccades described above, the human eye is in a constant state of vibration, oscillating back and forth at a rate of about 60 per second. These microsaccades are tiny movements, roughly 20 arcseconds in excursion and are completely imperceptible under normal circumstances. They serve to refresh the image being cast onto the rod cells and cone cells at the back of the eye. Without microsaccades, staring fixedly at something would cause the vision to cease after a few seconds since rods and cones only respond to a change in luminance."

I think that is what Ilya was talking about.

Ilya · #15 (**permalink**) 14-September-2007, 03:02 PM

Yes, that was it.

RBG · #16 (**permalink**) 25-April-2008, 02:55 AM

Nice find. I'm not saying it's not true but I'm not completely buying that they can act as a strobe. If those eye movements worked to freeze spoked wheels, they surely would interfere with 60 cycle NTSC television, and florescent lighting, etc, not to mention Mental Avenger's post #11 experiment. Not to mention most motion picture film people who have a religious belief in the eye/brain's natural ability to see things at 24 frames per second.

RBG

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