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Sathyasri Narasimhan, Srimant P. Tripathy, Brendan T. Barrett; Loss of positional information when tracking multiple dots: The role of memory. Journal of Vision 2004;4(8):361. doi: 10.1167/4.8.361.
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© ARVO (1962-2015); The Authors (2016-present)
Human observers can simultaneously track up to 5 dots, when presented with an array of dots moving in a random manner (Pylyshyn & Storm, 1988, Spatial Vision, 3, 179–197). However, the ability to detect deviations in straight-line trajectories is severely compromised when attending to two or more trajectories (Tripathy & Barrett, 2003, Journal of Vision, 3, 340a). Here we investigate the contribution of short-term memory to this loss of positional information. In previous experiments, the stimuli were several linear, non-parallel, left-to-right trajectories, each moving at the same speed. At the monitor's midline indicated by vertical markers (reached simultaneously by all trajectories), one of the trajectories (the target trajectory) deviated clockwise/anticlockwise. The angle of deviation was varied between trials using a method of constant stimuli. The observer reported the direction of deviation and thresholds were determined. Deviation thresholds rose steeply when the number of distractor trajectories was increased from 0 (typical threshold=2°) to 3 (typically over 40°). We modified the above stimulus so that all distractor trajectories disappeared on reaching the monitor's midline; only the target trajectory continued on the right half of the screen. Surprisingly, the observers still performed poorly; the task was impossible to perform when there were more than four distractor trajectories. Even for these simple linear/bilinear trajectories with the latter half of the target trajectory clearly identified, observers had little access to information regarding the first half of the target trajectory. This result represents a limitation of short-term memory, possibly as a consequence of attention not being directed to the first half of the target trajectory.
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