Position sense during imperceptibly slow movements

Proprioceptive input provides the nervous system with information related to body position and movement. This study investigated how normal humans perceive information related to the position of a limb when it is either at rest or moving very slowly, below the threshold for movement perception. Each subject's left hand, hidden from view, was horizontally translated in the frontal plane such that joint rotation was largely isolated to the shoulder. The translation speed was too slow for the subjects to detect movement. Subjects indicated their perception of hand position at 1- or 2-min intervals by pointing with the right index finger to the perceived location of the tip of the left middle finger. The constant error (i.e., bias) and variable error (i.e., SD of mean constant error) of each pointing movement was quantified. Partway through the trial, the direction of hand movement was reversed. In two additional experimental conditions, the subjects pointed at 1- or 2-min intervals at the stationary left hand, either when they assumed the hand would be moved or when they knew it would not. During slow movement, errors in the frontal and sagittal planes were found to be independent, and therefore, data analysis focused on the frontal plane, as this was the plane relevant to the subjects' task. All subjects performed the task sufficiently well to demonstrate a clear perception of hand location during the slow movement. The accuracy of perception was better when the left (target) hand was in its ipsilateral hemifield and, correspondingly, when the right hand pointed to its contralateral hemifield. There was no significant difference in constant error when the hand moved slowly, although there was a slightly higher variable error during slow movement than when the hand was stationary. Based on the similarity of results in trials with very slow and no hand movement, it was concluded that position-sense percepts are more accurately distinguished by the speed of movement rather than whether movement is occurring or not.