Motor control and position sense: action-perception coupling.

Abstract

According to previous studies, muscles become active in response to deviations from a threshold (referent) position of body segments. To test the hypothesis that corticospinal pathways set and reset the referent position in a task-specific way, we evaluated corticospinal influences at wrist positions established before and after voluntary motion as well as before and after involuntary motion elicited by the sudden removal of a load. Although tonic electromyogram (EMG) levels at pre- and post-unloading wrist positions were different, the corticospinal influences and thus the referent wrist position remained the same. These influences and the referent position changed, however, when subjects voluntarily moved their wrists to the other position. Thus, referent control strategies underlying the two types of motor actions are fundamentally different. We also tested the hypothesis that somatosensory afferents inform the brain about the deviation (P) of body segments from the centrally set referent position, R. To identify the actual position (Q) of body segments and form the position sense (PS), the central and afferent signals are combined. The PS rule was confirmed by demonstrating that: subjects are able to successfully reproduce involuntary changes in position elicited by sudden unloading of wrist flexors by making voluntary wrist movements; successfully reproduce elbow joint positions under different constant loads; subjects may not be aware of elbow flexion elicited by tendon vibration until a certain limit (no-motion illusion). In conclusion, R and P are additive components of PS and, contrary to conventional assumptions, PS is independent of sense of effort or efference copy.