The effect of frequency on the visual perception of relative phase and phase variability of two oscillating objects
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Relative phase has been studied extensively as a measure of interlimb coordination. Only two relative phases, namely 0° and 180°, are stably produced at the preferred frequency (∼1 Hz). When frequency is increased, movement at 180° becomes unstable and relative phase typically switches to 0°, which remains stable at higher frequencies. The current study was designed to investigate the perception of relative phase and of phase variability. Observers viewed two circles moving rhythmically in a computer display. Mean phases varied from 0° to 180° in 30° steps. Phase variability at each mean phase varied from 0° to 5°, 10°, and 15° phase standard deviation (SD). Frequency of oscillation was either 0.75 Hz or 1.25 Hz. One group of ten observers judged mean relative phase. Another group judged phase variability. As predicted, increase in frequency yielded an increase in perceived phase variability at 180° mean phase and other mean phases, but not at 0° mean phase. In contrast, increase in actual phase variability affected judgments of 0° mean phase most strongly. A second control experiment showed that the frequency effects were not produced by changes in display durations or frames per cycle of oscillation. The results are consistent with those in studies of interlimb coordination and indicate that understanding of interlimb coordination requires further investigation of phase perception.
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