Phi phenomenon

Demonstration of phi phenomenon using two black bars (SOA=102 ms, ISI=-51 ms)

The term phi phenomenon is used in a narrow sense for an apparent motion that is observed, if two adjacent optical stimuli are presented alternating with relative high frequency. In contrast to beta movement, seen at lower frequencies, no movement of the presented objects themselves is observed, but a diffuse, amorphous shadowlike something seems to jump in front of the objects and occludes them temporary. This shadow seems to have nearly the color of the background. This form of apparent movement was first described by Max Wertheimer in his habilitation thesis published 1912.

In a broader sense, particularly if the plural form phi phenomena is used, it applies also to all apparent movements that can be seen, if two adjacent optical stimuli are presented alternating. This includes especially the beta movement, which is important for motion illusion in cinema and animation. Actually Wertheimer applied the term “φ-movement” to all apparent movements described in his thesis as he introduced the term in 1912. But in recent years, mainly since the beginning of the 21st century, the term is used mainly for the shadowlike, objectless movement. For clarification also the term “pure phi” is used for the latter.

Experimental demonstration

„Magni- phi“, variant of the classical experimental arrangement with more than two elements.

The classical experiment, according to Wertheimer's, to reproduce the pure phi phenomenon uses two bars presented sequential on a screen, first on left side, then on the right. This sequence is repeated several times. If certain, relatively short intervals for the changeover are used and the distance of the objects suitable one can see the pure phi phenomenon.

Indeed it turned out to be much more difficult than with the beta movement to get stable and convincing results. To facilitate perception a more vivid experimental arrangement was designed using more than two objects. With this arrangement also called “Magni-phi” similar disks are arranged circularly and in a rapid sequence one of the disks is hidden in clockwise or counter-clockwise order. This seems to make it easier to observe this shadow-like movement. It is even more robust when changing parameters such as timing, size, intensity, number of disks or viewing distance.

Furthermore it was discovered that even with a arrangement according to the original using only two elements better results are possible, if a negative interstimulus interval (ISI) is used, that is, if the durations of the two elements are overlapping. Then the two objects are seen as stationary. That means, the viewer supposes unconsciously that the recurrence of the stimulus of one side means that the object previous displayed on this side reappeared and not, that the object of the opposite side has just moved there, as observed with beta movement. The crucial factor for this perception is the shortness of discontinuity of the stimulus on each side. This is supported by the observation that following two parameters have to be chosen properly to produce the pure phy phenomenon: First the absolute gap duration of each side must not exceed a value about 150 ms. Additionally the gap duration must not exceed 40% of the stimulus period.

History of research

In his thesis published 1912 Max Wertheimer introduced the symbol φ (phi) in the following way:

Gegeben sind sukzessiv zwei Objekte als Reize; diese werden empfunden; zuerst wird a gesehen, zuletzt b; zwischen ihnen war die ‚Bewegung von a nach b gesehen‘; ohne daß die entsprechende Bewegung resp. die raum-zeit-kontinuierlichen Zwischenlagen zwischen a und b wirklich als Reize exponiert gewesen wären. Der psychische Sachverhalt sei – ohne irgendeine Präjudiz – mit a φ b bezeichnet.

Two successive objects are given as stimuli; these are perceived; first a is seen, last b; between them the 'movement from a to b is seen'; without actually having exposed the corresponding movement respectively the time-space-continuous intermediate positions between a and b as stimuli. The physical issue will be denoted – without any prejudice – by a φ b.

Besides the “optimal movement” – later called beta movement – and partial movements of both objects Wertheimer described particularly a phenomenon he called “pure movement”. Concerning this he summarized the descriptions of his test persons in the following way:

Diese Fälle zeigten sich so, daß auch nicht etwa der Gedanke vorhanden war: ein Objekt habe sich hinüberbewegt; was von Objekten vorhanden war, war in den zwei Lagen gegeben; nicht eines oder eines von ihnen oder ein ähnliches betraf die Bewegung; sondern zwischen ihnen war Bewegung gegeben; nicht eine Objektbewegung. Auch nicht: das Objekt bewegt sich hinüber, ich sehe es nur nicht. Sondern es war einfach Bewegung da; nicht auf ein Objekt bezüglich.

These cases appeared in a way, that not even the thought was present: an object will have moved across; what was existing of objects was given in two positions; not one or one of them or a similar one accounted for the movement; but between them there was movement; not a movement of an object; Even not: the object moves across, I just don't see it. Instead it was just movement there; not regarding an object.

These observations Wertheimer attributed much importance, because in his opinion they proved that perception of movement was a direct sensation not necessarily deducted from the separate sensation of two optical stimuli in temporal an spatial distance. This aspect of his thesis may have been an important trigger to launch the so called Gestalt revolution.

Since mid of the 20th century confusion arose in the scientific literature what the phi phenomenon was about. One reason could be that the anglophone scientists had difficulties to understand Wertheimer's thesis published in German. Furthermore Wertheimer's thesis doesn't specify precisely for which parameters this “pure movement” was observed. Moreover its reproduction is relatively difficult. The influential standard work of Edwin Boring on sensory impression and perception, first published in 1942, probably made a decisive contribution to this confusion. Here Boring summarized the list of the observed phenomena of Wertheimer and sorted them ascending by the length of the interstimulus interval. But thereby he placed the phi phenomenon in the wrong position, namely as the phenomenon with the longest interval; with even longer intervals there's no perception of movement any more but one observes only two objects appearing successively.

This confusion has probably even contributed to “reinvention” of the pure phi phenomenon of other researches, for example as “omega motion”, “afterimage motion” or “shadow motion”.

See also

• Color phi phenomenon

External links

• Beta movement and Phi phenomenon.