User:Rainieday/Phi phenomenon

Reversed 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 reverse version of this motion, which is reversed phi illusion. 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 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 spacial 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 spacial 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.

Phi Phenomenon and 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. Phi phenomenon and Beta movement 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 unique and authentic one. Principles employed by our visual system to set the constraints are often relevant to simplicity and likelihood.