Why Do Wheels Sometimes Appear To Spin Backwards?

Post 5537                             Robert T. Gonzalez

Why Do Wheels Sometimes Appear To Spin Backwards?                                            Why Do Wheels Sometimes Appear To Spin Backwards?

You’ve seen this optical illusion in movies — but it happens when you’re watching in person too. What’s really going on?

Above: The reverse-rotation effect, as filmed by 1 Stop Auto Shop

It’s a familiar scene to most anyone with a television: The wheels of a forward-moving vehicle will appear at first to spin in one direction. The car puts on some speed and, as you would expect, its wheels rotate faster. But then, the something goes screwy. At a certain point, the spin of the wheels appears to slow, slow, slow. Then, ever so briefly, it stops. When it resumes, the spin is in the opposite direction. By appearance, the car should be moving backward – and yet, forward it rolls.

In Film And Television

This phenomenon is known as the “wagon-wheel” effect. If, like most people, you’re accustomed to seeing the wagon-wheel effect in movies or TV, its explanation is fairly straightforward: Cameras record footage not continuously, but by capturing a series of images in quick succession, at a specified “frame rate.” With many movie cameras, that rate is 24 frames per second. When the frequency of a wheel’s spin matches the frame rate of the camera recording it (say, 24 revolutions per second), each of the wheel’s spokes completes a full revolution every 1/24 seconds, such that it ends up in the same position every time the camera captures a frame. The result is footage in which the wheel in question appears motionless:

So when a wheel seems to spin in a direction opposite its actual rotation, it’s because each spoke has come up a few degrees shy of the position it occupied when it was last imaged by the camera. This is sometimes referred to as the reverse-rotation effect. If the spoke over-shoots, the wheel will appear to rotate in the right direction, but very, very slowly.

Why Do Wheels Sometimes Appear To Spin Backwards?

This, of course, is a simplified explanation. The appearance and strength of the effect also depend on things like the camera’s exposure time and the design of the wheel, itself. Take, for example, a wheel with 24-fold rotational symmetry. Assuming every one of its spokes looks identical, such a wheel will appear motionless to a camera shooting at 24 frames per second whether it’s rotating at 24 revolutions per second, 48 revolutions per second, or even one revolution per second. It doesn’t even have to be a wheel. Consider the footage shown here, in which a camera’s frame rate and shutter speed match perfectly with the rotational frequency of a helicopter’s blades. For the illusion to take shape, all that is required is that a repeating motion be visible intermittently. A similar phenomenon can therefore be achieved with a strobe light, giving rise to the related“stroboscopic effect”:

In Real Life

The wagon wheel effect, as seen on film and television, is easily explained. Less clear, however, is why people experience the the wagon-wheel effect not through a screen or by virtue of strobe lighting, but out in the real world, under constant lighting conditions. There are presently two competing hypotheses that account for this effect.

The first, proposed by neuroscientist Dave Purves and colleagues in a 1996 issue of Proceedings of the National Academy of Sciences, posits that humans perceive motion in a manner similar to a movie camera, i.e. not as one, continuous motion, but “by processing a series of visual episodes… the sequential presentation of discrete scenes.”

But in 2004, researchers led by neuroscientist David Eagleman demonstrated that test subjects shown two identical wheels spinning adjacent to one another often perceived their rotation as switching direction independently of one another. This observation is inconsistent with Purves’ team’s discrete-frame-processing model of human perception, which, reason suggests, would result in both wheels’ rotations switching direction simultaneously.

Why Do Wheels Sometimes Appear To Spin Backwards?

A “better” explanation for motion-reversal, Eagleman and his team conclude, is a form of “perceptual rivalry,” the phenomenon by which the brain generates multiple (or flat-out wrong) interpretations of a visually ambiguous scene. Classic examples of perceptual rivalry include the spatially ambiguous Necker cube, the hollow-face illusion, and – one of my personal favorites – the brain-bending silhouette illusion, famously illustrated by a spinning dancer that seems to switch directions at the drop of a hat.

[The Straight DopeLiveScienceGeekPNASVision Research]


I Am So Tempted to Say that These European Crows Are Racists

Post 5536    Annalee Newitz

I Am So Tempted to Say that These European Crows Are Racists                                  I Am So Tempted to Say that These European Crows Are Racists

In Europe, there are two groups of crows who are almost identical — except for for thing. Carrion crows are all black, while hooded crows have patches of gray. Each group prefers to mate with their own color. It sounds like what humans would call racism, but instead it’s a lesson in the limits of anthropomorphism.

Photo of a hooded crow by Nikolay Tretyakov

Two studies published this week in Science represent exhaustive analysis of the two crow groups, from their behavior to their genomes. What the researchers found was that the two groups occupied very-rigidly divided territories, with a thin “hybrid” zone running between them that was no more than 150 km wide at any point. In the map below, you can see the crow territories. Carrion crows are in the dark region in western Europe, and hooded crows are in the lighter central and eastern European areas. The dark line is the hybrid zone, where the two groups meet and often interbreed, producing fertile offspring.

I Am So Tempted to Say that These European Crows Are Racists

Still, the birds overwhelmingly prefer to mate with other birds of the same color. This reinforces the color differences between the two groups. Does that mean the two groups are becoming separate species?

It seems very unlikely. After exhaustive analysis, the two groups of researchers concluded that the crows were pretty much genetically identical. There were only three differences between them: color, the colors they preferred when it came to mating choices, and dominance behavior (carrion crows have slightly higher levels of hormones that govern aggression). Of the 82 DNA base pairs that differed between the two birds, reported evolutionary biologist Jochen Wolf and his team, “81 of them were concentrated in one genomic region coding for genes involved in coloration and visual perception.”

What this means is that the birds didn’t just differ in color — they differed in their abilities to perceive color. The more they choose birds of the same color, the more they evolved to perceive those color differences.

And this brings me back to my earlier assertion, which is that this is one of those scenarios in animal behavior where it’s extremely tempting to impose a human perspective and say that these crows are racists. They prefer to mate with their own color. The crows even have an actual color line running through their European habitat, dividing black from piebald. But the fact is that these crows’ mating choices have nothing to do with a slave economy, historical inequalities between groups, or cultural conflicts — all hallmarks of human racism.

There is undeniably a certain mystery behind why the birds evolved to be so acutely sensitive to color in mating choice. But they are hardly the first birds to fetishize feather colors for seemingly no reason other than sexual desire, as the peacock’s insane tail attests. The researchers suggest the two groups may have been separated by environmental conditions at some point in their evolution, and only reencountered each other relatively recently in species time. And their future is uncertain too. One group of researchers believe that the carrion will eventually overwhelm the hooded crows, while another believes the opposite.

But for now, these crows are testimony to the way small genetic differences can lead to scenarios that seem — from a human perspective — far more meaningful than they actually are.

Read a great summary of the two papers at Nature