Facilitation of tactile processing during action observation of goal-directed reach and grasp movements

Abstract

Our perception of sensory events can be altered by action, but less is known about how our perception can be altered by action observation. For example, our ability to detect tactile stimuli is reduced when our limb is moving, and task-relevance and movement speed can alter such tactile detectability. During action observation, however, the relationship between tactile processing and such modulating factors is not known. Thus, the current study sought to explore tactile processing at a task-relevant location during the observation of reaching and grasping movements performed at different speeds. Specifically, participants observed videos of an anonymous model performing movements at a slow (i.e., peak velocity [PV]: 155 mm/second), medium (i.e., PV: 547 mm/s), or fast speed (i.e., PV: 955 mm/s). To assess tactile processing, weak electrical stimuli of different amplitudes were presented to participants' right thumbs when the observed model was at their starting position and prior to any movement, or when the observed model's limb reached its PV. When observing slow movements, normalized perceptual thresholds were significantly lower/ better than for the pre-movement stimulation time. These data suggest that the movement speed can modulate tactile processing, even when observing a movement. Further, these findings provide seminal evidence for tactile facilitation at a task-relevant location during the observation of slow reaching and grasping movements (i.e., speeds associated with tactile exploration).

Distinct role of central predictive mechanisms in tactile suppression

“…Tactile suppression has been shown to depend on the time between stimulus and movement onset 10,15,45,47 and can be modulated by movement speed. 48,56,76 As we did not have a direct measure of movement speed, we used movement duration as an indirect measure of movement speed. In order to test potential differences in kinematic parameters across conditions, which might influence tactile suppression, we initially tested potential differences in these parameters as a function of condition, and if these parameters correlated with signal detection measures.…”

Tactile facilitation during actual and mere expectation of object reception

“…Sample size was first estimated at 15 using G*Power with the effect size calculated from the results of Ref. 46 (Cohen's d = 0.79), beta set to above 0.8 and alpha set to below 0.05. Then, due to the novelty of the experimental protocol, we recruited the higher sample sizes in comparable studies (e.g., Refs.…”

“…Then, due to the novelty of the experimental protocol, we recruited the higher sample sizes in comparable studies (e.g., Refs. 24,32,46 ), which also help deal with attrition. Indeed, two participants were excluded from statistical analyses because they detected the weakest stimulus intensity (i.e., 6 steps below the estimated threshold) 50% of the time.…”

“…And the trials surrounding step 0 allowing for a robust estimate of perceptual thresholds (see Ref. 46 for similar trial structure). Therefore, for each condition there were 68 trials.…”