The effects of voluntary movements on auditory-haptic and haptic-haptic temporal order judgments.

In two experiments we investigated the effects of voluntary movements on temporal haptic perception. Measures of sensitivity (JND) and temporal alignment (PSS) were obtained from temporal order judgments made on intermodal auditory-haptic (Experiment 1) or intramodal haptic (Experiment 2) stimulus pairs under three movement conditions. In the baseline, static condition, the arm of the participants remained stationary. In the passive condition, the arm was displaced by a servo-controlled motorized device. In the active condition, the participants moved voluntarily. The auditory stimulus was a short, 500Hz tone presented over headphones and the haptic stimulus was a brief suprathreshold force pulse applied to the tip of the index finger orthogonally to the finger movement. Active movement did not significantly affect discrimination sensitivity on the auditory-haptic stimulus pairs, whereas it significantly improved sensitivity in the case of the haptic stimulus pair, demonstrating a key role for motor command information in temporal sensitivity in the haptic system. Points of subjective simultaneity were by-and-large coincident with physical simultaneity, with one striking exception in the passive condition with the auditory-haptic stimulus pair. In the latter case, the haptic stimulus had to be presented 45ms before the auditory stimulus in order to obtain subjective simultaneity. A model is proposed to explain the discrimination performance.