Academic journal article Attention, Perception and Psychophysics

The Whole Moves Less Than the Spin of Its Parts

Academic journal article Attention, Perception and Psychophysics

The Whole Moves Less Than the Spin of Its Parts

Article excerpt

When individually moving elements in the visual scene are perceptually grouped together into a coherently moving object, they can appear to slow down. In the present article, we show that the perceived speed of a particular global-motion percept is not dictated completely by the speed of the local moving elements. We investigated a stimulus that leads to bistable percepts, in which local and global motion may be perceived in an alternating fashion. Four rotating dot pairs, when arranged into a square-like configuration, may be perceived either locally, as independently rotating dot pairs, or globally, as two large squares translating along overlapping circular trajectories. Using a modified version of this stimulus, we found that the perceptually grouped squares appeared to move more slowly than the locally perceived rotating dot pairs, suggesting that perceived motion magnitude is computed following a global analysis of form. Supplemental demos related to this article can be downloaded from app.psychonomic-journals.org/content/supplemental.

It is often said that the whole is greater than the sum of its parts. Is this true for motion perception? Can a global-motion percept appear to be faster or slower than the sum of local-motion inputs from which it derives? For example, when four pairs of dots are rotated continuously around their respective pair centers, as is shown in Figure 1, the percept alternates between one of four independently rotating dot pairs (local motion) and one of two overlapping squares that have dots at their corners and that appear to translate along overlapping circular trajectories (global motion; Anstis, 2003).1 When the stimulus changes from the local to the global percept, it appears to move more slowly, even though the dots themselves never change speed at the level of the stimulus. This implies that the perceived speed of the global-motion percept is not determined solely by the local speeds of the individual dots, which is consistent with the view that motion is computed in light of the outputs of a stage of form analysis (Ames, 1951; Caplovitz & Tse, 2007a, 2007b; Klopfer, 1991; Lorenceau & Shiffrar, 1992; Shiffrar, Li, & Lorenceau, 1995; Sinha & Poggio, 1996; Tse, 2006; Tse & Logothetis, 2002; Ullman, 1979). In the present article, we describe two psychophysical experiments that further examined the influence of perceived form on perceived motion.

Method

To investigate this "motion slowdown" effect, we effectively forced a local or global perceptual organization, rather than relying on the uncontrollable perceptual alternations that occur while one is viewing the original dot version of this stimulus (Anstis, 2003). To achieve this, we replaced each dot with an L-shaped stimulus (hereafter, "L").2 In the global stimulus configuration, the Ls were oriented to align in such a manner that they would consistently induce the global percept (shown on the left side of Figure 2). In the local configuration, the orientations of the Ls were chosen pseudorandomly, so as to induce the local percept of four independently rotating pairs.

To further bias this configuration toward the local percept, the starting orientation of each pair was randomized on every trial (right side of Figure 2). We thus created two versions of the stimulus that, although almost identical at the local level (observing the motion of only a single L would not reveal whether one was observing the local or global configuration), were radically different in their ability to induce the global percept.

Stimuli were white (67.02 cd/m2) on a gray (4.09 cd/m2) background, with a white (67.02 cd/m2) vertical bar separating the two sides of the display. The two line segments that constituted each L measured 0.5? of visual angle in length. Within a pair, the distance between Ls measured 0.8? (see Figure 2). The vertical distance between pairs was 3?, and the horizontal distance between pairs was 4? …

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