Have you ever had your scalp massaged with an orgasmatron? And then tried to re-instate the pleasure yourself but it just did not feel the same? It appears that a short time delay needs to be present between the movement of the hand and the experienced tactile sensation, in order for us to assign an external cause to the felt sensation. As a consequence, we enjoy it more as compared to when it is self-produced. Early physiological work demonstrated that tactile information is attenuated over the time-course of basic finger or forearm extensions, a phenomenon referred to as tactile attenuation (Chapman & Beauchamp, 2006). The most pervasive explanation for such an outcome is that tactile input may not be necessarily important to the executed movement (Bays & Wolpert, 2007).
In a recent study, we were interested whether this pattern of suppressive influences on tactile perception would be present for the movement preparation phase of ball-related movements, such as the basketball catching and throwing movements (Juravle & Spence, 2012). For this, we asked participants to detect a short temporal gap in an otherwise continuous vibratory stimulus applied to their wrist under three different conditions: a ball-throwing movement preparation (self-initiated movement), a ball-catching movement preparation (externally-generated movement), and a rest condition in which the participants only had to perform the perceptual tactile task and kept their hands at their side. In a subsequent experiment, we compared tactile sensitivity during the preparation of these self- versus externally-generated movements to their mere observation: Participants were required to perform the perceptual task while observing the initiation of a ball-catching or a ball-throwing from the experimenter’s side. Furthermore, we also tested the effect of different resting postures of the hands on tactile sensitivity by asking participants to perform the sensory task both with their hands at their sides, as well as with their hands holding the basketball.
When preparing a ball-catching movement, participants only exhibited a change in response bias, but no change in tactile sensitivity, as compared to the resting condition. On the other hand, preparing or only observing a self-initiated ball-throwing movement resulted in a significant drop in tactile sensitivity, as well as in a significant conservative shift in participants’ criterion. Our results thus indicate that what we feel is differentially affected by the preparation of self- versus externally-generated movements: We feel less when we prepare a self-initiated movement, but our sensitivity remains unchanged over the preparation period of reactive externally-triggered movements.
Furthermore, relevant to the readers of this blog is the result highlighting that our participants were significantly more inclined to say no target was present (reflecting a conservative criterion shift) while preparing to throw the ball, as compared to the resting ‘hands on the ball’ condition, but not when they kept their hands at their sides. This result indicates that we are more likely to experience movement-related sensory events if we have the specific body posture for that movement. Such a finding could potentially be of use for the training/rehabilitation programmes designed for chronic pain patients. In the light of such results, future research could test patients’ subjective pain ratings for novel/awkward postures when performing a given movement. An optimistic prediction would thus be that the ‘awkward’ movements with the painful body part (which benefits from patients’ full attention) would be perceived as less painful, as compared to the movements traditionally executed.
Georgiana Juravle
Crossmodal Research Laboratory, Department of Experimental Psychology, Oxford University, South Parks Road, Oxford, OX1 3UD, UK.
References
Bays PM, & Wolpert DM (2007). Computational principles of sensorimotor control that minimize uncertainty and variability. The Journal of physiology, 578 (Pt 2), 387-96 PMID: 17008369
Chapman CE, & Beauchamp E (2006). Differential controls over tactile detection in humans by motor commands and peripheral reafference. Journal of neurophysiology, 96 (3), 1664-75 PMID: 16775211
Juravle G, & Spence C (2012). Perceptual and decisional attenuation of tactile perception during the preparation of self- versus externally-generated movements. Experimental brain research, 223 (1), 109-20 PMID: 22948737