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Alex Holcombe, Piers Howe, Wei-Ying Chen; Load-dependent but short-range spatial interference in multiple object tracking. Journal of Vision 2013;13(9):1280. doi: 10.1167/13.9.1280.
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© ARVO (1962-2015); The Authors (2016-present)
Tracking performance declines when more targets must be tracked. Why? We previously documented one reason: temporal resolution is poorer when more targets are tracked (Holcombe & Chen, VSS 2012). For tracking one target, the temporal frequency threshold was 7 Hz - if distractors occupied former target locations more frequently than every 143 ms, tracking performance was poor. Tracking two targets yielded 4 Hz thresholds. Spatial resolution may also be poorer when more targets are tracked (Franconeri et al., 2008). We were interested in the range of such load-dependent spatial interference. Objects in our display were paired, with each pair sharing a circular trajectory centred on 8.5 deg. eccentricity, forming a "binary star". Two such binary stars were presented simultaneously with their trajectories separated by 1, 2, 3, 5, 7, or 9 deg. When there was only one target, speed thresholds for tracking were approximately 2.0 revolutions per second for all separations. When there were two targets, one in each binary star, thresholds were approximately 1.8 rps for all separations greater than 1 deg. At the separation of 1 deg however, the speed limit for tracking two targets fell to 1.6 rps, implicating short-range spatial interference that is greater when tracking two targets. Beyond 1-2 deg, the constant 1.8 rps limit suggests there is no distance-dependent spatial interference. In typical MOT displays, objects are free to approach and even touch each other. Our results suggest that in close encounters of 1 deg or less (where eccentricity is 8.5 deg), spatial interference causes tracking errors. At separations larger than that distance, spatial interference does not appear to be significant. More work is needed to determine the circumstances where spatial resolution rather than temporal resolution limits tracking, but the evidence here suggests that spatial resolution is confined to relatively short distances.
Meeting abstract presented at VSS 2013
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