Apparent motion

Anyone who has been to the cinema or watched television has experienced apparent motion. What is presented to the viewer is a rapid series of still images, but what is perceived is the illusion of continuous motion. Films are presented at a rate of 24 frames per second; this is known as the sample rate. Bruce et al. (1996, p. 187) made an important point about the relationship between apparent motion and real motion: "When the sample rate is high enough there is every reason to believe that 'real' (smooth) and 'apparent' (sampled) motion perception are effectively the same thing."

Apparent motion was shown under laboratory conditions by Wertheimer (1912), who was one of the Gestaltists (see Chapter 2). Two vertical lines in different spatial locations were presented alternately. When the delay between successive presentations was about one-twentieth of a second, observers often reported that there was one line that moved smoothly from place to place.

One of the main issues that needs to be resolved by the visual system in apparent motion is that of correspondence. This involves deciding which parts of successive still images belong to the same object in motion. Correspondence could be achieved by comparing each small part of successive images, but this would be very cumbersome with complex displays. For example, apparent motion can be created by using two large random-dot patterns which are identical except that dots in a square central position in one pattern are shifted to the left in the other pattern (discussed by Ramachandran & Anstis, 1986). When these two patterns are superimposed and presented in rapid alternation, a central square seems to move from side to side. As there are thousands of dots in each display, it seems improbable that the visual system meticulously compares each and every dot.

According to Ramachandran and Anstis (1986), the visual system focuses on certain features of a display when trying to detect correspondence. For example, the visual system seems good at detecting correspondences between areas of brightness and darkness (areas of low spatial frequency). A white square on a black background was presented for one-tenth of a second, and was replaced by a display with an outline square of the same size but coloured black on the left and a white circle on the left. The white square seemed to move towards the circle rather than towards the black square, suggesting that "the visual system tends to match areas of similar brightness in preference to matching sharp outlines" (Ramachandran & Anstis, 1986, p. 82).

The visual system prefers to perceive apparent motion in ways that would make sense in the real world. For example, we take account of the fact that objects in motion typically proceed along a straight path (the rule of inertia). This was shown in a two-stage experiment (Ramachandran & Anstis, 1986). In the first stage, two dots were presented rapidly at diagonal corners of an imaginary square, and were then replaced by identical dots in the opposite diagonal corners. About half the observers perceived two dots moving horizontally, with the other observers seeing the dots moving vertically. In the second stage, all the observers perceived two dots moving horizontally. The reason was that the display was embedded in the centre of a larger display in which two rows of dots moved horizontally creating an impression of linear movement in the larger display.

Ramachandran and Anstis (1986) argued that the visual system makes use of two other rules which affect decisions about correspondences or matches between successive images: the rule of rigidity and the rule of occlusion. According to the rule of rigidity, it is assumed that objects are rigid. Thus, if part of an object moves, all the rest of it moves as well. According to the rule of occlusion, an object continues to exist when it is hidden (or occluded) behind an intervening object. The relevance of these rules to apparent motion was shown using displays like those shown in Figure 3.8. The two displays were superimposed and then presented alternately. Four pie-shaped wedges are added and four are taken away, but what is seen is a white square moving right and left, occluding and uncovering discs in the background. This effect illustrates use of the rule of rigidity, because the dots within the square seem to move with it, even though in fact they remain stationary. The rule of occlusion is involved, because observers assume that the four circles remain intact, but that parts of them are occluded or partially obscured some of the time.

Theoretical accounts

The rules used by observers to detect correspondence and perceive apparent motion are largely based on their knowledge of regularities in the world and of the properties of objects. However, Ramachandran and Anstis (1986) argued that only relatively low-level processes are needed to produce the various effects they obtained. The experiments they described all involved rapid rates of stimulus presentation, and they claimed that it is unlikely that higher-level cognitive processes could have operated at those speeds. Ramachandran and Anstis also referred to neurobiological research (see Chapter 2) indicating that some nerve cells are sensitive to the motion of images with low spatial frequencies. These nerve cells may play a part in detecting correspondences at an early stage of visual processing.

Other theorists have argued that there is more than one kind of apparent motion. For example, Braddick (1980) proposed that apparent motion sometimes depends on the stimulation of low-level direction-selective cells. There is good evidence (Regan, Beverley, & Cynader, 1979) for direction-selective cells in the visual cortex responding mainly to a particular direction of movement. Particularly impressive evidence for their existence was obtained by Salzman et al. (1992). They studied the perceived direction of movement in random-dot displays in monkeys. Electrical stimulation of direction-selective cells biased the monkeys' perception of motion. When cells responding to rightward movement were stimulated, this increased the probability of the display appearing to move in a rightward direction.

According to Braddick (1980), apparent motion of the central square when two large random-dot patterns are superimposed and alternated (see earlier description) involves low-level, direction-selective cells. He referred to this as "short-range" motion. However, he assumed that apparent motion of line stimuli (e.g., Wertheimer, 1912, discussed earlier) may involve higher-level, more cognitive processes, and he termed this "long-range" motion.

Braddick (1980) discussed evidence indicating important differences between apparent motion with random-dot and line displays. Apparent motion with random-dot displays requires the stimuli to be much closer together than is the case to perceive apparent motion with line displays, and there need to be much shorter intervals of time between stimuli (under 100 milliseconds versus 300 milliseconds, respectively). In addition, apparent motion with random-dot displays is not observed when the two stimuli are presented to different eyes, but is still perceived with line displays.

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