Identifying the information for the visual perception of relative phase

The production and perception of coordinated rhythmic movement are very specifically structured. For production and perception, 0 degree mean relative phase is stable, 180 degrees is less stable, and no other state is stable without training. It has been hypothesized that perceptual stability characteristics underpin the movement stability characteristics, which has led to the development of a phase-driven oscillator model (e.g., Bingham, 2004a, 2004b). In the present study, a novel perturbation method was used to explore the identity of the perceptual information being used in rhythmic movement tasks. In the three conditions, relative position, relative speed, and frequency (variables motivated by the model) were selectively perturbed. Ten participants performed a judgment task to identify 0 degree or 180 degrees under these perturbation conditions, and 8 participants who had been trained to visually discriminate 90 degrees performed the task with perturbed 90 degrees displays. Discrimination of 0 degree and 180 degrees was unperturbed in 7 out of the 10 participants, but discrimination of 90 degrees was completely disrupted by the position perturbation and was made noisy by the frequency perturbation. We concluded that (1) the information used by most observers to perceive relative phase at 0 degree and 180 degrees was relative direction and (2) becoming an expert perceiver of 90 degrees entails learning a new variable composed of position and speed.