Spatial Modulation of Tactile Temporal-Order Judgments

We report a series of three experiments designed to examine the effect of posture on tactile temporal processing. Observers reported which of two tactile stimuli, presented to the left and right index fingers (experiments 1 – 3; or thumb, experiment 3), was perceived first while adopting one of two postures—hands-close (adjacent, but not touching) or hands-far (1 m apart)—in the dark. Just-noticeable differences were significantly smaller in the hands-far posture across all three experiments. In the first two experiments we compared hand versus foot responses and found equivalent advantages for the hands-far posture. In the final experiment the stimuli were presented to either the same or different digit on each hand (index finger or thumb) and we found that only when the same digit on each hand was stimulated was there an advantage for the hands-far posture. The finding that temporal precision was better with greater distance contradicts predictions based on attention-switching models of temporal-order judgments, and also contrasts with results from similar experimental manipulations in other modalities (eg vision). These results provide support for a rapid and automatic process that transforms the representation of a tactile stimulus from a skin-centred reference frame to a more external (eg body-centred or allocentric) one.