Today I took my bicycle out of storage. So what, I can hear you say. Ah, but before I jumped on it and pedaled away, I used it as a simple but effective means of examining the wagon-wheel illusion again.
Remember that Purves et al (1996) asserted in the prestigious _National Academy of Sciences_ that the wagon-wheel effect occurs with a DC light source and in daylight? That no less a deep thinker than Nobel-prize winner Francis Crick used this observation as evidence for "the discreteness of our conscious perception" (as cited by Pakarian & Yasamy, 2003), a conclusion with profound implications? And remember that Pakarian & Yasamy (2003) repeated Purves et al's research and reported that their results "are not replicable"? And, if you remember all that, possibly you also remember my call for a thousand wheels to spin and two thousand eyeballs to examine them? Well, here are the results from one wheel observed by two eyeballs. I first observed the rear wheel of my upended bicycle spinning in a vertical orientation under fluorescent light in my kitchen. I missed the effect at first because it's subtle and you have to know what to look for. Once I resolved that problem, I saw a clear wagon-wheel effect. It was best seen by pumping the pedal until the wheel spun rapidly, and then letting it coast. Under those conditions, I saw a reversal of the flickering of the spokes no less than five times as it slowed to a stop. The effect was reproducible. But when I took the bicycle outside and repeated the observations there, I could see no reversal. Not at 9:00 am, not at noon, not in the direction of the light, against it, or at 90 degrees to it. Nada. I did notice an immediately obvious difference viewing the spinning wheel under fluorescent light and in natural light. Under fluorescent, the flickering of the spokes was always visible; under natural light, as soon as the rotation speed increased, the spokes became invisible. Their prominence under fluorescence and disappearance under natural light is undoubtedly due to the stroboscopic effect of the fluorescecent light. At this point, it's worth quoting Purves et al's opening remarks: "We were stimulated to think further about this illusion and its interpretation when we noticed that the apparent backward rotation of wheels can also be seen in continuous light. Automobile wheel covers, airplane propellers, jet engine fans...rotating in daylight provide opportunities to observe this phenomenon". They also said "It should be noted that all the effects we describe were equally evident in sunlight, ruling out artifact associated with the dc power source". Well, perhaps in North Carolina, but not in Lennoxville, Quebec, and not in Tehran. As for the observations of Tom Alloway's students and those of Beth Benoit, in both cases moving cars were involved. At least in Beth's case, she herself was also in a moving car when she observed it, and I wonder if that was true in the student cases as well. A moving car may provide the flickering illumination which Pakarian's observations and mine suggest are essential to the viewing of the phenomenon. Only 999 wheels and 1,998 eyeballs to go. I hope we can get some further observations on this question. Can anyone confirm Purves' et al assertion of a wagon-wheel effect outdoors with a stationary observer? Stephen References Purves, D., et al (1996). The wagon wheel illusion in movies and reality. National Academy of Sciences (USA), 93, 3693-3697. Pakarian, P., & Yasamy, M. (2003). Wagon-wheel illusion under steady illumination: real of illusory? Perception, 32, 1`307-1310. ___________________________________________________ Stephen L. Black, Ph.D. tel: (819) 822-9600 ext 2470 Department of Psychology fax: (819) 822-9661 Bishop's University e-mail: [EMAIL PROTECTED] Lennoxville, QC J1M 1Z7 Canada Dept web page at http://www.ubishops.ca/ccc/div/soc/psy TIPS discussion list for psychology teachers at http://faculty.frostburg.edu/psyc/southerly/tips/index.htm _______________________________________________ --- You are currently subscribed to tips as: [EMAIL PROTECTED] To unsubscribe send a blank email to [EMAIL PROTECTED]
