The Phi Phenomenon

No one knows exactly what Wertheimer observed in 1910 through the train window as he was traveling from Vienna to begin a vacation. One version (Hunt, 2007) states that he noticed that the more distant telegraph poles, houses, and hilltops along the route seemed to speeding along with the train. He realized the ŌmovementĶ he was observing had to be somehow coming from his brain. To an observer outside the train, all of those stimuli would appear stationary. Regardless of what the inspiration was, it caused him to stop at the next station, Frankfurt, and buy a common toy available at the time, a zoetrope or stroboscope (see Fig. 12.x). The rapidly spinning slits made the slightly different images on the inside of the toy appear to move. He quickly dropped his vacation plans and went to the University of Frankfurt where one of his former teachers, Friedrich Schumann, was then working. He made laboratory space available along with a tachistoscope of his own

---------------Insert Fig. 12.x about here[Zoetrope toy]---------------

design. That allowed Wertheimer to present stimuli under accurate timing conditions far beyond those of the toy zoetrope. Schumann also volunteered two of his students, Wolfgang Kšhler and Kurt Koffka as research subjects. This lucky confluence of people

Marginal Definition: tachistoscope-a device that can display visual stimuli for extremely brief periods.

and events marked the beginning of Gestalt psychology as a new movement in psychology.

                        But, what exactly had Wertheimer found and why was it so important? Steinman, Pizlo, and Pizlo (2000) while teaching about Gestalt psychology found that their students were Ōskeptical about WertheimerÕs publication launching a revolution in perceptionĶ (p. 2257). Their students knew that motion pictures and nickelodeons had preceded the discovery of the phi phenomenon, so they wondered what was so new and unusual about it in 1912. The authors concluded that psychology textbook authors failed to describe the phi phenomenon correctly. Furthermore, they inferred that Boring (1942) had reversed the definitions of the phi phenomenon and its complementary partner, optimal movement (the beta phenomenon):

He [Boring] got only one thing wrong. Namely, the f-phenomenon is observed near simultaneity and not near successivity...The f-phenomenon is not observed when the switching speed is increased from successivity towards optimal movement (b). This, rather mysterious error in BoringÕs influential book probably led to the confusion about WertheimerÕs revolutionary phenomenon that is evident in most contemporary textbooks. (Steinman, Pizlo, & Pizlo, 2000, p. 2259)

More simply put, the phi phenomenon is not the apparent movement of an object to and fro caused by displaying two similar visual stimuli (e.g., dots or lines) alternately in a darkened room. Instead, the phi phenomenon that Wertheimer and his subjects observed was the movement of a spot between the two stimuli that occurred when the stimuli were presented nearly simultaneously. Such spots were not real; they were created by the brain. Furthermore, observers saw both stimuli and the moving spot at all times. Filip J. Pislo has developed a JAVA applet that displays the phi phenomenon nicely. It can be viewed online at:

                        WertheimerÕs article on the phi phenomenon, while groundbreaking, provided only the sparsest description of the phenomenon itself. His lectures and other verbal communication were much more influential in spreading the finding. Wertheimer was always at his best while speaking and often confessed that he had trouble writing about his ideas. Also, he allowed his ideas to sit, unpublished, for years before finally putting them in written form. Students who heard his lectures, both in Germany and later in the United States, fell into two distinct camps. They either became avid disciples or were completely lost. Wertheimer published the least of the three original Gestalt psychologists but was always considered to be the fountainhead of the movement. For example, Koffka (1935, pp. 53-54) wrote:

Wertheimer had just completed his experiments on the perception of motion in which Kšhler and I had served as the chief observers...on that afternoon he said something which impressed me more than anything else, and that was his idea about the function of a physiological theory in psychology, the relation between consciousness and the underlying physiological processes, or in our new terminology, between the behavioural and the physiological field. To state it in these new terms, however, is not quite fair, because this very statement was only made possible by WertheimerÕs idea; before, nobody thought of a physiological or, for that matter, of a behavioural field. [italics in the original]

WertheimerÕs discovery of the phi phenomenon was more than the simple elucidation of a new finding. He also sought to explain the physiological basis for it and ruled out, experimentally, older explanations that argued that eye movements were the cause. The physiological mechanism he proposed, short circuits in the cortex, has not stood the test of time. Indeed, modern psychology has yet to provide a universally accepted physiological explanation for the phi phenomenon. From the beginning Gestalt psychologists saw themselves as experimenters and posited a causal relationship between the molar phenomena they studied and yet-to-be discovered neural mechanisms. Further, their vision for Gestalt psychology went far beyond the area of perception alone. Wertheimer, late in his career, was more interested in Gestalt analysis of thinking and problem solving. Still, the impact of Gestalt psychology on the study of perception was immense. WertheimerÕs (1923) early research was especially so.