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Kurt Debono, Alexander Schütz, Karl Gegenfurtner; Illusory bending of a pursuit target sheds light on early direction estimation. Journal of Vision 2011;11(11):532. doi: 10.1167/11.11.532.
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To pursue a target of interest, it must first be separated from background motion. If various sources provide information about direction, pursuit has been shown to initially average vectors, and then adopt a winner-take-all strategy. Here we show that a peripheral marker can modulate initial perceptual and pursuit integration before target direction is resolved, thus causing illusory target bending. The marker can either be used as an initial cue or provide a fixed reference point for the illusion to take place.
Subjects pursued a central target that moved outwards at 10 deg/s along a straight line for 900ms in a random direction. A 100% coherent RDK background moved within a central circular window (radius 10 deg) at the same speed in a direction that differed up to 25° from target direction. A marker consisting of a small 2-deg line was presented at the outer edge of the circular window throughout the trial. At the beginning of each trial a central fixation spot was presented together with the marker. Subjects pressed a button to initiate motion and were instructed to pursue the target with their eyes.
When the marker was aligned with target direction, observers perceived the target as moving along a straight line. However, when it was aligned with the RDK, they observed a strong illusory bend in target trajectory. Initial target direction was perceived to be closer to the marker and RDK directions. After about 200 ms, the target direction seemed to bend towards its true direction. Pursuit showed a similar but smaller effect.
We present a new illusion that demonstrates how the perceived motion path of a pursuit target can be updated online as different sources of direction information from the target, background motion and peripheral cues are processed.
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