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Harriet A Allen, Robert F Hess, Tim Ledgeway; Discriminating the direction of randomly positioned contrast-defined motion. Journal of Vision 2003;3(9):532. doi: 10.1167/3.9.532.
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© ARVO (1962-2015); The Authors (2016-present)
We investigated whether positional uncertainty affected observe' ability to discriminate the direction of luminance-defined and contrast-defined motion. If the mechanisms that detect contrast-defined motion can't be simultaneously monitored, not knowing the position of contrast-defined motion will severely affect performance.
Random dot kinematograms were presented on a circular field (radius10 deg) of low contrast 2D binary noise. Dots were either brighter (luminance-defined) or higher contrast (contrast-defined) than the noise and moved at 3 deg/sec. In a circular target area (radius 1deg) the dots moved either up or down. The remaining dots, surrounding the target area, moved randomly. The target area was centered 2 deg from fixation and there were no dot-density cues to its location. Observers discriminated the direction of motion in the target area (2AFC method) when they knew its position and when it was randomly in 1 of 4 positions. Experiment 1 measured the modulation depth (contrast-defined patterns) or contrast (luminance-defined patterns) required to discriminate motion direction. Experiment 2 measured the number of coherently moving dots required to perform the same task. Both experiments were carried out with stimulus durations of 250ms and 100ms
Thresholds for the motion in randomly positioned areas ranged from 1.1 to 3.4 times the thresholds for the motion in the known position. In experiment 1 the increase in threshold was slightly larger at the shorter duration. For each condition and observer the size of the effect was almost identical for luminance-defined and contrast-defined motion. Mechanisms for contrast-defined motion are not differentially affected in their ability to process motion signals of uncertain position compared with those for luminance-defined motion. Previous findings showing poor performance with multiple patches of contrast-defined motion must reflect some other deficiency in the mechanisms for contrast-defined motion.
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