The effect of frequency on the visual perception of relative phase and phase variability of two oscillating objects

Relative phase has been studied extensively as a measure of interlimb coord ination. Only two relative phases, namely 0 degrees and 180 degrees, are st ably produced at the preferred frequency (similar to1 Hz). When frequency i s increased, movement at 180 degrees becomes unstable and relative phase ty pically switches to 0 degrees, which remains stable at higher frequencies. The current study was designed to investigate the perception of relative ph ase and of phase variability. Observers viewed two circles moving rhythmica lly in a computer display. Mean phases varied from 0 degrees to 180 degrees in 30 degrees steps. Phase variability at each mean phase varied from 0 de grees to 5 degrees, 10 degrees, and 15 degrees 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 variabili ty. As predicted, increase in frequency yielded an increase in perceived ph ase variability at 180 degrees mean phase and other mean phases, but not at 0 degrees mean phase. In contrast, increase in actual phase variability af fected judgments of 0 degrees mean phase most strongly. A second control ex periment showed that the frequency effects were not produced by changes in display durations or frames per cycle of oscillation. The results are consi stent with those in studies of interlimb coordination and indicate that und erstanding of interlimb coordination requires further investigation of phas e perception.