Temporal onset-order discrimination through the tactual sense: effects of frequency and site of stimulation.

This research extends the study of temporal resolution of the tactual sensory system through measurements of temporal-onset order discrimination for continuous tonal signals addressing (a) the effects of frequency separation of the two stimuli whose onset orders are to be discriminated and (b) the effects of redundant coding of frequency and site of stimulation on performance. Sinusoidal signals were presented either at two separate digits (thumb and index finger of the left hand) or at a single site of stimulation (left index finger) using a multifinger tactual stimulation system. Measurements were obtained using a one-interval two-alternative forced choice procedure in which each interval consisted of the random-order presentation of two different stimuli with roving values of amplitude and duration. Thresholds were estimated from psychometric functions of d' as a function of stimulus-onset asynchrony (SOA). On average, temporal onset-order thresholds were larger for one-finger conditions (mean SOA of 74.8 ms) than for two-finger conditions (mean SOA of 48.5 ms) and decreased as frequency separation increased, particularly for single-site presentation. Redundant coding of frequency and site of stimulation resulted in higher resolution by a factor of 1.5 compared to frequency alone and by a factor of 1.2 compared to site alone.