Dynamic temporal modulation of somatosensory processing during reaching

Abstract

Sensorimotor control of human action integrates feedforward policies that predict future body states with online sensory feedback. These predictions lead to a suppression of the associated feedback signals. Here, we examine whether somatosensory processing throughout a goal-directed movement is constantly suppressed or dynamically tuned so that online feedback processing is enhanced at critical moments of the movement. Participants reached towards their other hand in the absence of visual input and detected a probing tactile stimulus on their moving or static hand. Somatosensory processing on the moving hand was dynamically tuned over the time course of reaching, being hampered in early and late stages of the movement, but, interestingly, recovering around the time of maximal speed. This novel finding of temporal somatosensory tuning was further corroborated in a second experiment, in which larger movement amplitudes shifted the absolute time of maximal speed later in the movement. We further show that the release from suppression on the moving limb was temporally coupled with enhanced somatosensory processing on the target hand. We discuss these results in the context of optimal feedforward control and suggest that somatosensory processing is dynamically tuned during the time course of reaching by enhancing sensory processing at critical moments of the movement.

Tactile suppression stems from sensation-specific sensorimotor predictions

“…These probes served as a proxy for the sensory states associated with the movement. 12,18 Consistent with previous work, 3,6,12,16 we observed increased detection thresholds and hampered detection precision of tactile sensations around movement onset compared to detection at rest. Critically, congruent conditions led to stronger suppression of detection thresholds, but not of precision.…”

“…Tactile suppression is commonly tested by applying externally-generated tactile probes on a limb that is actively moving, a well-established paradigm that allows quantifying tactile suppression at various body parts and timepoints. 1,3,6 Although this phenomenon has been attributed to predictive mechanisms, such as internal forward models, 4 other accounts claim that it may stem from an unspecific gating that does not distinguish between predictable and unpredictable sensory states, but instead cancels out any tactile probes arising on the moving limb. 5,7 Despite evidence that a movement-induced reduction in tactile sensitivity is accompanied by a downregulation of neural activity in the primary somatosensory and motor cortices already before movement onset, 8 whether and how this phenomenon is related to prediction and expresses itself on the perceptual level is still obscure.…”

“…With the present results we cannot determine the extent to which gating may contribute to the observed suppression effects. As tactile suppression does not depend on the strength of reafferent input 33 and, importantly, can fall off completely during movement, 6 even at moments when transmission of muscle spindle afferences is increased, 34 some additional modulatory mechanism seems to be involved. Our present findings support the idea that tactile suppression stems from a specific prediction about the future sensory states of the movement, providing novel evidence for a predictive mechanism involved in this phenomenon.…”

“…16 This suggests that suppression depends on an interplay between predictive and feedback signals. 6 However, no study hitherto has distinguished predictive from general suppression mechanisms, leaving the phenomenon open to different interpretations. Our findings can reconcile computational approaches of sensorimotor control with tuning of sensory processing, and thus contribute to resolving a debate about the origins of tactile suppression.…”

“…Our present findings support the idea that tactile suppression stems from a specific prediction about the future sensory states of the movement, providing novel evidence for a predictive mechanism involved in this phenomenon. Thus, unspecific gating may not be a competing explanation for the copious accounts of action-related suppression of externally-generated stimulations, 6,12,19,30 but rather a separate mechanism operating in addition to predictive suppression.…”