Visual perception of mean relative phase and phase variability

Perception of relative phase and phase variability may play a fundamental r ole in interlimb coordination. This study was designed to investigate the p erception of relative phase and of phase variability and the stability of p erception in each case. Observers judged the relative phasing of two circle s rhythmically moving on a computer display. The circles moved from side to side, simulating movement in the frontoparallel plane, or increased and de creased in size, simulating movement in depth. Under each viewing condition , participants observed the same displays but were to judge either mean rel ative phase or phase variability. Phase variability interfered with the mea n-relative-phase judgments, in particular when the mean relative phase was 0 degrees. Judgments of phase variability varied as a function of mean rela tive phase. Furthermore, the stability of the judgments followed an asymmet ric inverted U-shaped relation with mean relative phase, as predicted by th e Haken-Kelso-Bunz model.