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The effects of different orientations of in-phase and counter-phase backgrounds on horizontal motion discrimination

56.4001, Tuesday, 19-May, 2:45 pm - 6:45 pm, Pavilion
Session: Motion Perception: Local and higher order

Andrew Silva1, Zili Liu1; 1Department of Psychology, University of California, Los Angeles

Introduction: When a paired-dot stimulus is locally balanced such that the dots within each pair travel in opposite directions and in close spatial proximity over a short distance (counter-phase motion), a flicker-like percept occurs (Qian, Andersen, & Adelson, 1994). However, if the motion of one dot per pair is reversed such that both dots travel in the same direction (in-phase motion), a globally transparent percept occurs (Lu, Qian, & Liu, 2004). Here, we examined how discrimination sensitivity to horizontal motion is affected by different orientations of a superimposed in-phase or counter-phase background. A reasonable hypothesis is that near-horizontal background orientations should result in better overall performance than steeper orientations, as the former case creates stimuli with greater numbers of directional signals near the target direction. However, it is unknown whether this expected pattern is modulated by the local balancing of the counter-phase background. Method: Fourteen participants discriminated the direction of a horizontally moving field of target dots. This field was presented simultaneously with 676 pairs of background dots, all exhibiting in-phase or counter-phase motion along a common orientation between 0 and 90 degrees from horizontal. A 2-down, 1-up staircase measured the number of target dots required to achieve 71% accuracy. Results: Background orientations between 0 and 30 degrees from horizontal resulted in low overall thresholds and a relatively small difference between in-phase and counter-phase thresholds. Steeper background orientations resulted in increased thresholds, though the rate of increase was considerably faster for the counter-phase background. Conclusions: A detrimental effect of local balancing was found. However, this effect was less apparent for near-horizontal background orientations, suggesting that a global target motion can perceptually capture any similarly-oriented background dots in locally balanced as well as locally unbalanced backgrounds.

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