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John Greenwood, Mark Edwards; Seeing multiple global directions: A maximum capacity limit of three. Journal of Vision 2008;8(6):1027. doi: 10.1167/8.6.1027.
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© ARVO (1962-2015); The Authors (2016-present)
Though much is known about the global-motion stage, relatively little research has addressed the number of global directions that can be perceived simultaneously. Recently, a restricted capacity has been reported for the detection of transparent motion, which occurs when multiple global directions are present within the same spatial region. No more than two transparent-motion signals can be detected simultaneously when defined by direction differences, though three signals can be seen when distributed across multiple independent global-motion systems. The aim of the present study was to determine whether this three-signal capacity reflects the specific mechanisms of transparent-motion detection or a more general restriction on the detection of global-motion signals, particularly in light of the higher capacities reported for visual attention and multiple object tracking. Using both transparent motion and spatially segregated stimuli, observers were required to indicate which of two temporal intervals contained the most signal directions. Simultaneous processing was ensured through brief durations (200ms) and comparisons between n and n+1 directions, eg. 3 vs. 4. When spatially segregated signals were interleaved in patches across the entire stimulus, no more than two directions were seen, as with transparent motion. In contrast, separating the signal directions into distinct spatial regions allowed the detection of up to three signals. Importantly, the signal intensities required to detect multiple directions did not vary across these signal arrangements. This suggests that the two-signal capacity results from signal-to-noise pooling across the entire stimulus, while the higher capacity for spatially distinct signal regions reflects independent global pooling within each signal region. Together, these results demonstrate that the capacity limit of three is not restricted to transparent-motion detection. Rather, it represents a strict upper limit for global-motion processing that is insensitive to manipulations of signal intensity.
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