Enhancement and Suppression of Tactile Signals During Reaching
The perception of tactile stimuli presented on a moving hand is systematically suppressed. Such suppression has been attributed to the limited capacity of the brain to process task-irrelevant sensory information. Here, we examined whether humans do not only suppress movement-irrelevant but also enhance in parallel movement-relevant tactile signals when performing a goal-directed reaching movement. Participants reached to either a visual (LED) or somatosensory target (thumb or index finger of their unseen static hand) and discriminated 2 simultaneously presented tactile stimuli: a reference stimulus on the little finger of their static hand and a comparison stimulus on the index finger of their moving hand. Thus, during somatosensory reaching the location of the reference stimulus was task-relevant. Tactile suppression, as reflected by the increased points-of-subjective-equality (PSE) and just-noticeable-differences (JND), was stronger during reaching to somatosensory than visual targets. In Experiment 2, we presented the reference stimulus at a task-irrelevant location (sternum) and found similar suppression for somatosensory and visual reaching. This suggests that participants enhanced the sensation of the reference stimulus at the target hand during somatosensory reaching in Experiment 1. This suggestion was confirmed in Experiment 3 using a detection task in which we found lower detection thresholds on the target hand during somatosensory but not during visual reaching. We postulate that humans can flexibly modulate their tactile sensitivity by suppressing movement-irrelevant and enhancing movement-relevant signals in parallel when executing a reaching movement.