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Spinning wheels deceive, so do fishermen, fast trip to Mars

Q: I just read your answer to why rotating wheels appear to go backwards in movies. It’s because the camera snaps different frames 24 times each second, right? But I notice this in real life. Does the eye take a certain number of snaps every second too?? (Hussain, Montreal, Canada)

Scientists put different spins on the phenomenon of the backwards-whirling wagon wheel. [© Copyright Jeff Adams, used with permission]

A: You’re not the only one that sees the "backward spinning wagon wheels" illusion in real life — outdoors in the steady sunshine and not watching a flickering movie. Almost all people do especially if they look at the spinning wheel for a prolonged time (13 seconds to 8 minutes, in one experiment).

Neuroscientists, however, disagree on what causes the illusion we experience — some agreeing with your snapshot idea. One states, "We normally see motion, as in movies, by processing a series of visual episodes." (Purves et al.) In other words, snapshots. Another calls it "batch-like effects in vision." (Crick and Koch.) Again, snapshots.

Others don’t agree with the snapshot explanation. Kline, Holcombe, and Eagleman think it’s our tricked perception — a phenomenon called "perceptual rivalry" — that causes the illusion. In that case, our minds interpret the same scene in different ways — alternating between the two different interpretations.

Stare a bit and the box appears to "come out of the screen" in one of two directions. Click for an animated version — with pink and green beams, a flying dog and a diving man!

For example, we can look at a cube doodle (see figure) and perceive it ‘coming out of the page’ in one of two directions. Typically, our brains interpret the cube in one direction, then the other — switching back and forth. We’ve all experienced the phenomenon. Click the figure for a cool animated version.

"There are two striking observations here: first your brain never sees both interpretations at once, and second, your interpretation changes, even though nothing on the screen has changed," says David Eagleman, neurobiology professor at the University of Texas Medical School at Houston .

These observations form the basis of Eagleman’s argument for perceptual rivalry over the snapshot theory. Eagleman and his student performed an experiment to decide which theory is correct. They spun a drum and propped up a mirror beside it so observers saw the spinning drum and its reflected image at the same time.

The snapshot hypothesis predicts that both the drum and its mirrored image will appear to reverse simultaneously since the brain discretely samples the entire field of vision, including both the drum and its reflection. On the other hand, perceptual rivalry says either drum will appear to switch direction, independently of the other.

Five (out of five) observers saw the drum and its reflection reverse independently. That is, they saw one drum appear to reverse direction for a short time while the other continued to rotate in the true direction. The illusion switched.

When we look (a good long look) at spinning wheels outdoors, Eagleman thinks that our perception switches back and forth between the real forward motion and an illusion of reversed motion. Why? When we see a pattern (like a rotating drum) moving in one direction, it "activates the proper motion detectors in our brain but it also tickles motion detectors that code for the opposite direction," says Eagleman. The resulting competition between detectors causes our perception to switch back and forth.

Further Reading:

Vision Research: Illusory motion reversal is caused by rivalry, not by perceptual snapshots of the visual field by Keith Kline, Alex O. Holcombe, and David M. Eagleman.

Proceedings of the National Academy of Science, Neurobiology: The wagon wheel illusion in movies and reality by Dale Purves, Joseph A. Paydarfar, and Timothy J. Andrews

Visual Illusions and Neurobiology by David M. Eagleman

Q: What’s the biggest great white shark ever caught? (Bill, Grimsby, United Kingdom)

Great white sharks come at least 20 feet (6.4 m) long [NOAA]

A: The (reliable) champ is a female 20-foot (6.1-m) long great white caught off the maritime Canadian province, Prince Edward Island, in August 1983. Her statistics are recorded in the scientific literature.

Tracking down the biggest great white is an elusive fish-story fraught business. Some others:

Further Reading:

ReefQuest Centre for Shark Research: Largest shark ever caught by R. Aidan Martin

Q: How long does it take sunlight to reach Mars? How do I mathematically find this out? (Ashlea, Doylestown, Pennsylvania)

Mars — a mere 13 minutes away (for light) [NASA]

A: It takes a sunbeam somewhere between 11 and 14 minutes to reach Mars — depending where Mars is in her orbit around the Sun. The planet’s closest distance to the Sun is 206 million kilometers and the farthest distance is 250 million kilometers. Light travels at 300,000 kilometers per second.

That compares with the 8 minutes (on the average) for sunlight to reach our nearer planet, Earth.

To find out how long sunlight takes to get to Mars, we divide the distance by the speed of light.

(Answered Jan. 28, 2005)