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Monday, 11 June 2012
The Phi Effect Is Not the Beta Effect!

illusionScience is not immune to historic errors that can be passed along for decades. One such error was the confusion of the phi effect with the beta effect, which persisted until Robert M. Steinman and his colleagues published their clarification in 2000.

The phi effect was first described in Experimental Studies on the Seeing of Motion, a book published in 1912 by Max Wertheimer, one of the fathers of Gestalt psychology. The problem was that in his book, Wertheimer did not describe the conditions for the appearance of the phi effect precisely. He said that this phenomenon occurs when two lines are projected on a screen in very close chronological succession, thus creating the impression (under certain observation conditions that he left undefined) that a fuzzily defined area the same colour as the background is moving between these two lines.

In his book, Wertheimer distinguishes this this “pure apparent movement” from another type of apparent movement called beta. The beta illusion of movement, like the phi, is caused by the presentation of fixed images in succession, but at a slower rate, similar to the rate at which fixed images are displayed in a movie or a video to create the impression of continuous movement.

However, most textbooks and web sites that discuss apparent movement in movies, whether from the standpoint of the cinema or from that of psychology (including the present web site, until now), still attribute this apparent movement to the phi effect and not to the beta effect. In his article, Steinman points out one possible source of this confusion.

The source in question is another book, Sensation and Perception in the History of Experimental Psychology, published by E.G. Boring in 1942. Boring’s descriptions of Wertheimer’s observations of the phi and beta effects are correct, but Boring errs when he states that the phi effect is observed when the images succeed each other at a relatively slow rate. In reality, it is observed when they do so at a relatively fast rate, close to the one at which the two images are seen simultaneously. Because Boring’s book was so influential in the years that followed, Steinman believes that it was this strange error on Boring’s part, as well as the lack of clarity in Wertheimer’s original description, that made subsequent generations of researchers mistake the beta effect for the phi effect.

Students reading this blog may be interested to learn that it was when Steinman was trying to reproduce the surprising phi effect in front of his students to convince them of its existence that he realized Boring’s error. And it was when I was preparing to give a lecture about vision to a group of students that I came across Steinman’s article. Thus we see that in the course of their jobs, teachers can sometimes uncover the errors of the past, and not just perpetuate them!

a_lien Phi is not beta, and why Wertheimer’s discovery launched the Gestalt revolution
a_lien Magni-phi and related phenomena
i_lien Phi phenomenon
i_lien Beta movement

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