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Srimant P. Tripathy, Brendan T. Barrett, Sathyasri Narasimhan; Gross distortions in perveived trajectories when tracking multiple dots. Journal of Vision 2004;4(8):368. doi: 10.1167/4.8.368.
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Our ability to detect a deviation in a straight-line trajectory is severely compromised when we are attending to two or more straight-line trajectories simultaneously (Tripathy & Barrett, 2003, Journal of Vision, 3, 340a). Here we investigate the perceived shape of the deviating trajectory in the presence of other non-deviating distractor trajectories. The stimuli were several linear non-parallel trajectories, all moving left-to-right, at the same speed. At the monitor's midline indicated by vertical markers (reached simultaneously by all trajectories), one of the trajectories deviated clockwise/anticlockwise, while the others continued undeviated. Observers described the perceived distortions in the shape of the deviating trajectory (Tripathy & Barrett, Perception, in press). Subsequent experiments objectively measured the perceived distortions. In spite of knowing that the deviating trajectory was bilinear and that the deviation occurred at the midline, most observers reported the following distortions: a. The perceived location of the deviation was shifted in the direction of motion. b. Immediately following the deviation observers overestimated the angle of deviation. c. The deviating trajectory appeared to accelerate at the point of deviation. d. Towards the end of the trajectory observers underestimated the angle of deviation. The misestimation of the initial and final angles resulted in the bilinear trajectory appearing grossly S-shaped. We suggest that mislocalisation of the deviation results from attention being directed to the wrong trajectory at the instant of deviation. Distortions b, c and d result from the visual system's efforts to compensate for the initial mislocalisation (a).
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