6.2.2 Stroboscopic apparent motion

**Figure 6.15:** The *zoetrope* was developed in the 1830s and provided stroboscopic apparent motion as images became visible through slits in a rotating disc.
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Nearly everyone on Earth has seen a motion picture, whether through a TV, smartphone, or movie screen. The motions we see are an illusion because a sequence of still pictures is being flashed onto the screen. This phenomenon is called stroboscopic apparent motion; it was discovered and refined across the 19th century. The zoetrope, shown in Figure 6.15 was developed around 1834. It consists of a rotating drum with slits that allow each frame to be visible for an instant while the drum rotates. In Section 1.3, Figure 1.24 showed the Horse in Motion film from 1878.

**Figure 6.16:** The *phi phenomenon* and *beta movement* are physiologically distinct effects in which motion is perceived [355,312]. In the sequence of dots, one is turned *off* at any give time. A different dot is turned *off* in each frame, following a clockwise pattern. At a very low speed (2 FPS), beta movement triggers a motion perception of each on dot directly behind the *off* dot. The on dot appears to jump to the position of the *off* dot. At a higher rate, such as 15 FPS, there instead appears to be a moving hole; this corresponds to the phi phenomenon.
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Why does this illusion of motion work? An early theory, which has largely been refuted in recent years, is called persistence of vision. The theory states that images persist in the vision system during the intervals in between frames, thereby causing them to be perceived as continuous. One piece of evidence against this theory is that images persist in the visual cortex for around

ms, which implies that the

FPS (Frames Per Second) is the slowest speed that stroboscopic apparent motion would work; however, it is also perceived down to

FPS [312]. Another piece of evidence against the persistence of vision is the existence of stroboscopic apparent motions that cannot be accounted for by it. The phi phenomenon and beta movement are examples of motion perceived in a sequence of blinking lights, rather than flashing frames (see Figure 6.16). The most likely reason that stroboscopic apparent motion works is that it triggers the neural motion detection circuitry illustrated in Figure 6.12 [206,214].