Demonstration of phi phenomenon using two black bars (SOA=102ms, ISI=−51ms)
The term phi phenomenon is used in a narrow sense for an apparent motion that is observed if two nearby optical stimuli are presented in alternation with a relatively high frequency. In contrast to beta movement, seen at lower frequencies, the stimuli themselves do not appear to move. Instead, a diffuse, amorphous shadowlike something seems to jump in front of the stimuli and occlude them temporarily. This shadow seems to have nearly the color of the background.[1]Max Wertheimer first described this form of apparent movement in his habilitation thesis, published 1912,[2] marking the birth of Gestalt psychology.[3]
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 nearby optical stimuli are presented in alternation. This includes especially beta movement, which has been regarded as the illusion of motion in cinema and animation, [4][5] although it can be argued that beta movement indicates long-range apparent motion rather than the short-range apparent motion seen in film.[6] Actually, Wertheimer applied the term "φ-phenomenon" to all apparent movements described in his thesis when he introduced the term in 1912, the objectless movement he called "pure φ".[2] Nevertheless, some commentators assert that he reserved the Greek letter φ for pure, objectless movement.[7][8]
Experimental demonstration
"Magni-phi" variant of the classical experimental arrangement with more than two elements
Wertheimer's classic experiments used two light lines or curves repeatedly presented one after the other using a tachistoscope.[9] If certain, relatively short, intervals between stimuli were used, and the distance between the stimuli was suitable, then his subjects (who happened to be his colleagues Wolfgang Köhler and Kurt Koffka[10]) reported seeing pure "objectless" motion.[9]
However, it turns out to be difficult to demonstrate phi stably and convincingly. To facilitate demonstrating the phenomenon, 21st-century psychologists designed a more vivid experimental arrangement using more than two stimuli. In this demonstration, called "Magni-phi," identical disks are arranged in a circle and, in a rapid sequence, one of the disks is hidden in clockwise or counter-clockwise order. This makes it easier to observe the kind of shadow-like movement Wertheimer discovered. The Magni-phi demonstration is robust to changes of parameters such as timing, size, intensity, number of disks, and viewing distance.[9]
Furthermore, the phenomenon may be observed more reliably even with only two elements if a negative interstimulus interval (ISI) is used (that is, if the periods during which the two elements are visible overlap slightly). In that case, the viewer may see the two objects as stationary and suppose unconsciously that the reappearance of the stimulus on one side means that the object previously displayed in that position has reappeared and not, as observed with beta movement, that the object from the opposite side has just moved to a new position. The crucial factor for this perception is the shortness of discontinuity of the stimulus on each side. This is supported by the observation that two parameters have to be chosen properly to produce the pure phi phenomenon: first the absolute duration of the gap on each side must not exceed about 150ms., and second, the duration of the gap must not exceed 40% of the stimulus period.[1]
History of research
In his 1912 thesis, Wertheimer introduced the symbol φ (phi) in the following way:[2]
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 a phenomenon he called "pure movement." Concerning this, he summarized the descriptions of his test subjects as follows:
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 has moved across; what was existing of objects was given in two positions; neither one nor the other of them nor a similar one accounted for the movement; but between them there was movement; not a movement of an object. Not even: the object moves across, I just don't see it. Instead, it was just movement there; not regarding an object.
Wertheimer attributed much importance to these observations because, in his opinion, they proved that movement could be perceived directly and was not necessarily deduced from the separate sensation of two optical stimuli in slightly different places at slightly different times.[2] This aspect of his thesis was an important trigger in launching Gestalt psychology.[9]
Starting in the mid-20th century, confusion arose in the scientific literature as to exactly what the phi phenomenon was. One reason could be that the anglophone scientists had difficulties understanding Wertheimer's thesis, which was published in German. Wertheimer's writing style is also idiosyncratic.[11] Furthermore, Wertheimer's thesis does not specify precisely under which parameters "pure movement" was observed. Moreover, it is difficult to reproduce the phenomenon. Edwin Boring's influential history of the psychology of sensation and perception, first published in 1942, contributed to this confusion.[12] Boring listed the phenomena Wertheimer had observed and sorted them by the length of the interstimulus interval. However, Boring placed the phi phenomenon in the wrong position, namely as having a relatively long inter stimulus interval. In fact, with such long intervals, subjects do not perceive movement at all; they only observe two objects appearing successively.[9]
This confusion has probably contributed to the "rediscovery" of the phi phenomenon under other names, for example, as "omega motion," "afterimage motion," and "shadow motion."[1]
Reverse phi illusion
As apparent phi movement is perceived by human’s visual system with two stationary and similar optical stimuli presented next to each other exposing successively with high frequency, there is also a reversed version of this motion, which is reversed phi illusion.[13] Reverse phi illusion is the kind of phi phenomenon that fades or dissolves from its positive direction to the displaced negative, so that the apparent motion human perceive is opposite to the actual physical displacement. Reverse phi illusion is often followed by black and white patterns.
It is believed that reverse phi illusion is indeed brightness effects, that it occurs when brightness-reversing picture moving across our retina.[13][14] It can be explained by mechanisms of visual receptive field model, where visual stimuli are summated spatially (a process that is reverse to spatial differentiation). This spatial summation blurs the contour to a small extent, and thus changes the brightness perceived. Four predictions are confirmed from this receptive field model. First, foveal reverse-phi should be broken down when the displacement is greater than the width of foveal receptive fields. Second, reverse phi illusion exists in the peripheral retina for greater displacements than in the fovea, for receptive fields are greater in the peripheral retina. Third, the spatial summation by the receptive fields could be increased by the visual blurring of the reversed phi illusion projected on a screen with defocus lens. Fourth, the amount of reversed phi illusion should be increasing with the decrease of displacement between positive and negative pictures.
Indeed, our visual system processes forward and reversed phi phenomenon in the same way. Our visual system perceives phi phenomenon between individual points of corresponding brightness in successive frames, and phi movement is determined on a local, point-for-point basis mediated by brightness instead of on a global basis.[14]
Neural mechanism underlying sensitivity to reversed phi phenomenon[15]
T4 and T5 motion detectors cells are necessary and sufficient for reversed phi behavior, and there are no other pathways to produce turning responses for reversed phi motion. [citation needed]
Tangential cells show partial voltage response with the stimulation of reversed phi motion. [citation needed]
There is substantial responses for reversed-phi in T4 dendrites, and marginal responses in T5 dendrites. [citation needed]
Phi phenomenon and beta movement
Example of beta movement
Phi phenomenon has long been confused with beta movement; however, the founder of Gestalt School of Psychology, Max Wertheimer, has distinguished the difference between them in 1912. While Phi phenomenon and Beta movement can be considered in the same category in a broader sense, they are quite distinct indeed.
Firstly, the difference is on neuroanatomical level. Visual information is processed in two pathways, one processes position and motion, and the other one processes form and color. If an object is moving or changing position, it would be likely to stimulate both pathways and result in a percept of beta movement. Whereas if the object changes position too rapidly, it might result in a percept of pure movement such as phi phenomenon.
Secondly, phi phenomenon and beta movement are also different perceptually. For phi phenomenon, two stimuli A and B are presented successively, what you perceive is some motion passing over A and B; while for beta movement, still with two stimuli A and B presented in succession, what you perceive would be an object actually passing from position A to position B.
The difference also lies on cognitive level, about how our visual system interprets movement, which is based on the assumption that visual system solves an inverse problem of perceptual interpretation. For neighboring stimuli produced by an object, the visual system has to infer the object since the neighboring stimuli do not give the complete picture of the reality. There are more than one way for our visual system to interpret. Therefore, our visual system needs to put constraints to multiple interpretations in order to acquire the unique and authentic one. Principles employed by our visual system to set the constraints are often relevant to simplicity and likelihood.[16]
Hassenstein–Reichardt detector model
Hassenstein–Reichardt detection model
The Hassenstein–Reichardt detector model is considered to be the first mathematical model to propose that our visual system estimates motion by detecting a temporal cross-correlation of light intensities from two neighboring points, in short a theoretical neural circuit for how our visual system track motion. This model can explain and predict phi phenomenon and its reversed version.[15][17] This model consists two locations and two visual inputs, that if one input at one location is detected, the signal would be sent to the other location. Two visual inputs would be asymmetrically filtered in time, then the visual contrast at one location is multiplied with the time-delayed contrast from the other location. Finally, the multiplication result would be subtracted to obtain an output.
Therefore, two positive or two negative signals would generate a positive output; but if the inputs are one positive and one negative, the output would be negative. This corresponds to the multiplication rule mathematically.
For phi phenomenon, motion detector would develop to detect a change in light intensities at one point on the retina, then our visual system would compute a correlation of that change with a change in light intensities of a neighboring point on the retina, with a short delay.[18]
Reichardt model
The Reichardt model[17] is a more complex form of the simplest Hassenstein–Reichardt detector model, which is considered to be a pairwise model with a common quadratic nonlinearity. As Fourier method is considered to be linear method, Reichardt Model introduces multiplicative nonlinearity when our visual responses to luminance changes at different element locations are combined.[19] In this model, one photoreceptor input would be delayed by a filter to be compared by the multiplication with the other input from a neighboring location. The input would be filtered two times in a mirror-symmetrical manner, one before the multiplication and one after the multiplication, which gives a second-order motion estimation.[17][20] This generalized Reichardt model allows arbitrary filters before the multiplicative nonlinearity as well as filters post-nonlinearity.[17] Phi Phenomenon is often regarded as first-order motion, but reversed phi could be both first-order and second-order, according to this model. [21]
Persistence of vision is the optical illusion that occurs when the visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been described as "retinal persistence", "persistence of impressions", simply "persistence" and other variations. A very commonly given example of the phenomenon is the apparent fiery trail of a glowing coal or burning stick while it is whirled around in the dark.
In visual perception, an optical illusion is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.
Gestalt psychology, gestaltism, or configurationism is a school of psychology and a theory of perception that emphasises the processing of entire patterns and configurations, and not merely individual components. It emerged in the early twentieth century in Austria and Germany as a rejection of basic principles of Wilhelm Wundt's and Edward Titchener's elementalist and structuralist psychology.
Max Wertheimer was a psychologist who was one of the three founders of Gestalt psychology, along with Kurt Koffka and Wolfgang Köhler. He is known for his book, Productive Thinking, and for conceiving the phi phenomenon as part of his work in Gestalt psychology.
The term beta movement is used for the optical illusion of apparent motion in which the very short projection of one figure and a subsequent very short projection of a more or less similar figure in a different location are experienced as one figure moving.
The Ternus illusion, also commonly referred to as the Ternus Effect, is an illusion related to human visual perception involving apparent motion. In a simplified explanation of one form of the illusion, two discs, are shown side by side as the first frame in a sequence of three frames. Next a blank frame is presented for a very short, variable duration. In the final frame, two similar discs are then shown in a shifted position. Depending on various factors including the time intervals between frames as well as spacing and layout, observers perceive either element motion, in which L appears to move to R while C remains stationary or they report experiencing group motion, in which L and C appear to move together to C and R. Both element motion and group motion can be observed in animated examples to the right in Figures 1 and 2.
The term illusory motion, also known as motion illusion or "apparent motion", is an optical illusion in which a static image appears to be moving due to the cognitive effects of interacting color contrasts, object shapes, and position. The stroboscopic animation effect is the most common type of illusory motion and is perceived when images are displayed in fast succession, as occurs in movies. The concept of illusory motion was allegedly first described by Aristotle.
Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult problem from a computational perspective, and difficult to explain in terms of neural processing.
The lilac chaser is a visual illusion, also known as the Pac-Man illusion. It consists of 12 lilac, blurred discs arranged in a circle, around a small black, central cross on a grey background. One of the discs disappears briefly, then the next, and the next, and so on, in a clockwise direction. When one stares at the cross for at least 30 seconds, one sees three illusions
A gap running around the circle of lilac discs;
A green disc running around the circle of lilac discs in place of the gap; and
The green disc running around on the grey background, with the lilac discs having disappeared in sequence.
The color phi phenomenon is the fact that, when apparent motion is induced between objects with different colors, the color of the apparently moving object abruptly changes midway along the path. It is a perceptual illusion described by psychologists Paul Kolers and Michael von Grünau in which a disembodied perception of motion is produced by a succession of still images. The color phi phenomenon is a more complex variation of the phi phenomenon. Kolers and von Grünau originally investigated the phenomenon in response to a question posed by the philosopher Nelson Goodman, who asked what the effect of the color change would have on the phi phenomenon.
Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the 20th century, but they are present in art dating to the Middle Ages. Gaetano Kanizsa’s 1976 Scientific American paper marked the resurgence of interest in illusory contours for vision scientists.
Infant vision concerns the development of visual ability in human infants from birth through the first years of life. The aspects of human vision which develop following birth include visual acuity, tracking, color perception, depth perception, and object recognition.
The flash lag illusion or flash-lag effect is a visual illusion wherein a flash and a moving object that appear in the same location are perceived to be displaced from one another. Several explanations for this simple illusion have been explored in the neuroscience literature.
In visual perception, the kinetic depth effect refers to the phenomenon whereby the three-dimensional structural form of an object can be perceived when the object is moving. In the absence of other visual depth cues, this might be the only perception mechanism available to infer the object's shape. Being able to identify a structure from a motion stimulus through the human visual system was shown by Hans Wallach and O'Connell in the 1950s through their experiments.
Stuart M. Anstis is a professor emeritus of psychology at the University of California, San Diego, in the United States.
In human visual perception, the visual angle, denoted θ, subtended by a viewed object sometimes looks larger or smaller than its actual value. One approach to this phenomenon posits a subjective correlate to the visual angle: the perceived visual angle or perceived angular size. An optical illusion where the physical and subjective angles differ is then called a visual angle illusion or angular size illusion.
Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.
The watercolor illusion, also referred to as the water-color effect, is an optical illusion in which a white area takes on a pale tint of a thin, bright, intensely colored polygon surrounding it if the coloured polygon is itself surrounded by a thin, darker border. The inner and outer borders of watercolor illusion objects often are of complementary colours. The watercolor illusion is best when the inner and outer contours have chromaticities in opposite directions in color space. The most common complementary pair is orange and purple. The watercolor illusion is dependent on the combination of luminance and color contrast of the contour lines in order to have the color spreading effect occur.
Stereoscopic motion, as introduced by Béla Julesz in his book Foundations of Cyclopean Perception of 1971, is a translational motion of figure boundaries defined by changes in binocular disparity over time in a real-life 3D scene, a 3D film or other stereoscopic scene. This translational motion gives rise to a mental representation of three dimensional motion created in the brain on the basis of the binocular motion stimuli. Whereas the motion stimuli as presented to the eyes have a different direction for each eye, the stereoscopic motion is perceived as yet another direction on the basis of the views of both eyes taken together. Stereoscopic motion, as it is perceived by the brain, is also referred to as cyclopean motion, and the processing of visual input that takes place in the visual system relating to stereoscopic motion is called stereoscopic motion processing.
Ensemble coding, also known as ensemble perception or summary representation, is a theory in cognitive neuroscience about the internal representation of groups of objects in the human mind. Ensemble coding proposes that such information is recorded via summary statistics, particularly the average or variance. Experimental evidence tends to support the theory for low-level visual information, such as shapes and sizes, as well as some high-level features such as face gender. Nonetheless, it remains unclear the extent to which ensemble coding applies to high-level or non-visual stimuli, and the theory remains the subject of active research.
References
1 2 3 Vebjørn Ekroll, Franz Faul, Jürgen Golz: Classification of apparent motion percepts based on temporal factors. In: Journal of Vision. Volume 8, 2008, Issue 31, pp. 1–22 (online).
1 2 3 4 Max Wertheimer: Experimentelle Studien über das Sehen von Bewegung. Zeitschrift für Psychologie, Volume 61, 1912, pp. 161–265 (onlineArchived 2018-11-23 at the Wayback Machine ;PDF-Datei; 8,61MB).
↑ Friedrich Kenkel: Untersuchungen über den Zusammenhang zwischen Erscheinungsgröße und Erscheinungsbewegung bei einigen sogenannten optischen Täuschungen. In: F. Schumann (ed.): Zeitschrift für Psychologie. Volume 67, Leipzig 1913, p. 363
↑ Martha Blassnigg: Time, Memory, Consciousness and the Cinema Experience: Revisiting Ideas on Matter and Spirit. Edision Rodopi, Amsterdam/New York 2009, ISBN90-420-2640-5, p. 126 (online).
↑ Anderson, Joseph; Anderson, Barbara (1993). "The Myth of Persistence of Vision Revisited". Journal of Film and Video. 45 (1): 3–12. JSTOR20687993.
1 2 3 4 5 Robert M. Steinman, Zygmunt Pizlob, Filip J. Pizlob: Phi is not beta, and why Wertheimer's discovery launched the Gestalt revolution. In: Vision Research. Volume 40, 2000, pp. 2257–2264 (online).
↑ Smith, Barry (1988). "Gestalt Theory: An Essay in Philosophy". In Smith, Barry (ed.). Foundations of Gestalt Theory. Vienna: Philosophia Verlag. pp.11–81. Archived from the original on 2012-02-22. Retrieved 2019-10-12.
↑ Shipley, Thorne, ed. (1961). Classics in Psychology. New York: Philosophical Library. pp.1032, footnote 1. Retrieved 2019-10-22.
↑ Edwin Boring: Sensation And Perception In The History Of Experimental Psychology. Appleton-Century-Crofts, New York 1942 (online).
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