And while we're on the subject of optical illusions, I've done a follow-up to our discussion of the wagon wheel effect.
If you recall, Purves et al (1996) reported that the wagon wheel effect, contrary to expectation, occurs with a continuous light source, such as under artificial illumination with a filtered DC source, and in daylight. They suggested this revealed important information concerning the way in which the visual system processes information. Alas, Pakarian & Yasamy (2003) reported a failure to replicate their finding. I also was unable to see the wagon-wheel effect using my spinning bicycle wheel outdoors, although it was readily apparent indoors under fluorescent light. So I wrote to Dale Purves and received a cordial reply. However, he asked that I not post it here, as he was understandably concerned that this might encourage numerous others to bug him by e-mail on the issue. He had no objection to my summarizing its contents, however. Briefly, he was dismissive of the P & Y effort, stating that the phenomenon has been repeatedly confirmed. In his view, the question is no longer whether the effect exists in continuous light, but why. However he did seem to backtrack on the claim in his paper that the effect is easily seen under natural circumstances in daylight (with "automobile wheel covers, airplane propellers, jet engine fans", and with a radial pattern on a record player). Instead he now says that the speed "has to be just right" and therefore is best observed under laboratory conditions. Also, in response to the claim in his paper that "all the effects we observed were equally evident in sunlight", he told me that this meant when illuminated only by daylight coming through the window. Given all of this, I still wonder why P & Y and I failed to observe it under continuous daylight illumination. Purves believes his systematic observations under "highly-filtered DC" are the definitive ones, but since these must be derived from converted AC, perhaps there is just enough flicker left to induce the illusion. So I trust observations in daylight more, and he seems not to have systematically studied the phenomenon using this form of illumination. I also question the premise that I didn't see the effect because the speed is critical. In my attempt, I let the spinning wheel coast to a stop, and so observed over a wide range of speed. The effect appeared consistently at a set of critical speeds under fluorescent but not under daylight. Another possibility, given the failure of at least one person on our thread to fail to see the "rotating snakes" illusion, is that there exists individual, perhaps ethnic differences in ability to see the wagon-wheel effect in daylight. While 11 of 12 observers in North Carolina saw the illusion, 0 of 15 saw it in Iran. But where does that leave me, neither North Carolinian nor Iranian? So I think the individual differences explanation is unlikely. But I would like to hear from anyone, anywhere, who can reliably observe the effect under daylight. Sometimes discrepancies between different observers can lead to important insights (and sometimes to the firing of astonomers). I hope someone is looking into this. Actually, Dr. Purves gave me the name of someone who is, and I think I'll try bugging him as well. Stephen References Pakarian, P., & Yasamy, M. (2003). Wagon-wheel illusion under steady illumination: real or illusory? Perception, 32, 1307-1310 (available from the author at [EMAIL PROTECTED] Purves, D. et al (1996). The wagon wheel illusion in movies and reality. Proceedings of the National Academy of Science, 93, 3693- 3697 (available at www.pnas.org/cgi/reprint/93/8/3693.pdf ___________________________________________________ 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]
