Academic journal article Canadian Journal of Experimental Psychology

The Effects of Motion Adaptation and Disparity in Motion Integration

Academic journal article Canadian Journal of Experimental Psychology

The Effects of Motion Adaptation and Disparity in Motion Integration

Article excerpt

Abstract Two experiments examined the effects of pattern vs component adaptation on motion integration in stimuli with or without disparity. Observers adapted to either downward pattern motion, downward component motion, or a grey screen, and were then tested with plaids containing either crossed, uncrossed or zero binocular disparity, moving downward. In a second experiment, the same test conditions were employed following adaptation to upward pattern motion. The total amount of time that coherence or transparent sliding was perceived was measured. Adaptation to component motion increased the amount of perceived coherent motion whereas adaptation to pattern motion decreased it. The second experiment revealed that adaptation to the upward-moving pattern had no effect on perceived coherence. The results demonstrate the complex nature of the interaction between depth and motion mechanisms in motion integration.

Under prolonged inspection, a moving squarewave plaid' yields bistable percepts: Perception switches between separate gratings sliding over one another in their respective directions, and a coherent plaid moving in a third direction. The "separate" percept implies that each grating is processed by an appropriate direction-sensitive mechanism and that both are operating simultaneously. This is also referred to as the stage of component motion. The "coherent" percept implies a further stage of processing, whereby the two component gratings are integrated into a new pattern, a plaid, which moves in a composite direction, different from the two original directions. This stage has been referred to as pattern motion, and models have been developed to describe the perceived pattern direction (Adelson & Movshon, 1982; Movshon, Adelson, Gizzi, & Newsome, 1985; Wilson, Ferrera, & Yo, 1992; Jasinschi, Rosenfeld, & Sumi, 1992; Kim & Wilson, 1993).

In previous studies (von Grunau & Dube, 1993), we have shown that unambiguous adaptation to either component or pattern motion can alter the proportion of coherence in a bistable plaid stimulus. Thus, adaptation to a component stimulus resulted in a reduction of time for the percept of component motion (sliding motion). Similarly, adaptation to a pattern stimulus reduced the time for the percept of pattern motion (coherent motion). Moreover, an analysis of the length of the individual episodes of coherent and sliding motion within a test run revealed that exposure to one of the two types of motion resulted in a decrease of the length of only the corresponding episodes, leaving the others unaffected. This behaviour implicates a high degree of independence between two underlying processes which nonetheless must be connected to each other at some level, so that the stronger one at any one time will take precedence and determine perception (switching behaviour).

One parameter found in the past to modify the bistability in these kinds of plaid stimuli is binocular disparity (Adelson & Movshon, 1984; Trueswell & Hayhoe, 1993). We have demonstrated previously that perceived coherence is systematically related to the amount of disparity between the two gratings: As disparity was increased, there was a subsequent decrease in the proportion of coherence (von Grunau, Dube, & Kwas, 1993). It was also found that adaptation to a stationary plaid pattern, containing crossed or uncrossed disparity between the two gratings, resulted in a relative increase of coherent motion in an ambiguous test plaid. This means that exposure to two surfaces separated in depth had the effect of decreasing the time that a bistable motion stimulus was seen as two patterns moving in two different directions at two different depth planes. Rather, it was seen more frequently as a single pattern with one motion direction lying in one depth plane.

The present experiments explored further the ways in which disparity parameters can modulate the motion integration process, a fact that was implicated by the two previous sets of findings. …

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