Academic journal article Attention, Perception and Psychophysics

Simultaneity and Sequence in the Perception of Apparent Motion

Academic journal article Attention, Perception and Psychophysics

Simultaneity and Sequence in the Perception of Apparent Motion

Article excerpt

Motion perception usually is accompanied by the phenomenological impression of sequence as objects move through successions of locations. Nonetheless, there is accumulating evidence that sequential information is neither necessary nor sufficient for perceiving motion. It is shown here that apparent motion is specified by counterchange rather than sequence-that is, by co-occurring toward- and away-from-background changes at two spatial locations, regardless of whether the changes are simultaneous or sequential. Motion is perceived from the location of the toward to the location of the away change, even when the changes occur in reverse temporal order. It is not perceived for sequences of away or toward changes, as would be expected if motion were specified by onset or offset asynchronicity. Results previously attributed to onset and offset asynchrony are instead attributable to onsets and offsets occurring in close temporal proximity at the same location. This was consistent with units for detecting away and toward changes that are temporally biphasic; that is, they are excited by changes in one direction and inhibited by immediately preceding or immediately following changes in the opposite direction. These results are accounted for by a model for counterchange-specified motion entailing the biphasic detection of toward and away changes.

When an object moves through space, it occupies a series of spatial locations along its trajectory, providing a strong phenomenological impression of sequence. It is perhaps for this reason that motion-specifying stimulus information has been thought to be sequential. Accordingly, classical models have emphasized the detection of sequential information for pairs of visual events, incorporating temporal delays in order to bring the activations produced by the events into temporal coincidence (e.g., Reichardt, 1961). Consistent with motion being specified by sequences of stimulus events, experimental researchers have provided evidence for effects of stimulus onset asynchrony (the time between the onsets of successive events) and interstimulus interval (the time between the offset of one event and the onset of the next event) on the perception of apparent motion (Burt & Sperling, 1981; Kahneman & Wolman, 1970; Kolers, 1964; Korte, 1915; Lakatos & Shepard, 1997; Wertheimer, 1912).

The present article furthers our investigation of an alternative to sequence-determined motion perception: namely, that motion can be specified by a pattern of stimulus change (Gibson, 1968) entailing counterchange-that is, oppositely signed changes in luminance or texture contrast at two locations. The experiments distinguish between counterchange and sequential information and provide further evidence that motion energy detection is not responsible for the perception of single-element apparent motion between pairs of simultaneously visible surfaces (motion energy extraction is not precluded as the basis for motion perception under other circumstances).

In standard apparent motion (Figure 1A), a surface is discontinuously displaced from one location to another; so, conceivably, motion could be specified by the sequence of locations that the surface occupies. However, the displacement also creates counterchange. For lighterthan- background surfaces, luminance decreases to the background value at the location initially occupied by the surface and increases away from the background value at the location newly occupied by the displaced surface. In generalized apparent motion (Figures 1B and 1C), the surface is simultaneously visible at both of its locations, but the pattern of toward and away changes is the same (Hock, Kogan, & Espinoza, 1997; Johansson, 1950). Surfaces do not change location for these stimuli, so the perception of apparent motion does not require detecting a sequence of location changes. Exner (1875) similarly showed that apparent motion can be perceived for successive flashes that are too close to each other to be spatially resolved. …

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