Introduction to apparent movement

Unlike the real movement, apparent movement is produced by stimuli separated both in space and time. As Steinman et al. (2000) state, a great body of research on the apparent movement is misled by an influential classic book written by Boring (1942) called Sensation and Perception in the History of Experimental Psychology. An error caused by the rather deliberate use of words and phrases in Weltheimer’s original German text has caused lots of trouble for later generations of researchers. In one paragraph of his book (p.595), Boring manages to mislocate Weltheimer's revolutionary φ-phenomenon at the low end of the temporal switching frequency range causing most of the later researchers to use wrong definitions especially about the pure apparent movement.

Often φ is mixed up with its cousin β-phenomenon that means optimal movement, and even more often these two are simply referenced with the general term apparent movement without emphasizing the fundamental difference between them. To put it short, β means an illusion where two or more stimuli are presented one after another in a cyclic way with certain ISI (interstimulus interval) and SD (stimulus duration) making the stimuli to seem to step from one place to another. Note that the pace of the stepping in β is slow enough to be perceived (optimal apparent movement). When creating φ, the pace of the stepping is speeded up so that the perception of movement seems to be continuous (pure apparent movement). Especially characteristic for φ is that other physical stimulus properties do not affect the perception of movement as long as those are above the absolute threshold level.

Thus, φ is found between the simultaneity of stimuli presentation and optimal movement β. Not to make it too simple, there is also a phenomenon called partial movement that can be found between β and successive presentation of stimuli. In the partial movement, the stimuli are perceived to move a short distance closer to each other while not giving the impression of a stimulus changing its place. To really make this all understandable, both β and φ are animated in Figures 1 & 2 below.

Figure 1. Optimal visual movement (β)	Figure 2. Pure visual movement (φ)

The illusion of apparent movement has been replicated with all three major exploratory senses, namely touch, vision and hearing. Steinman et al. (2000) say that φ is probably impossible to notice when it is produced by only single pairs of stimuli but quite easy to perceive when the stimuli are being presented repeatedly. This is because φ is perceived when the stimuli are stationary but flickering making the flicker a necessary condition for perceiving φ at least in visual condition. Moreover, in visual φ the color of _φ_-effect is always same as the color of the background and it has no shape because it does not have contour.

In my quest for vibrotactually mediated touch, the need to present the stimuli repeatedly in order to create a sensation of continuous movement can cause a methodological difference between the real act of touching and its intended vibrotactile equivalent at least for some brief moving gestures. However, some initial experiments have to be carried out to clarify the effects of different kinds of properties for vibrotactile feedbacks.

Next, I'll concentrate on SensorySaltation? - the phenomenon of perceiving a ghost stimuli spreading between two or more real stimulus location, originally called as "cutaneous rabbit"..

Steinman, R. M., Pizlo, Z. & Pizlo, F. J. (2000) Phi is not beta, and why Wertheimer's discovery launched the Gestalt revolution: a minireview. Vision Research, 40, 2257-2264. (available at http://www2.psych.purdue.edu/Magniphi/)