The perception of the position of an unseen limb: Investigation of the effect of thixotropic conditioning on drift and accuracy.

Proprioceptive judgements can be divided into two broad categories: low-level and high-level. Low-level judgements of limb position require a person to detect, discriminate or match the position of a body part, whereas high-level judgements require a person to report the position of an unseen body part relative to the external world. It has been suggested that muscle thixotropy - the influence of recent contraction or stretch on the passive properties of a muscle - impacts both the accuracy of low-level judgements of limb position and the degree to which these judgements drift over time. However, high-level proprioceptive judgements of upper limb position and the degree to which they drift over time may not be affected by thixotropy. This was investigated here. Twenty-five healthy adult participants made visual judgements about the perceived position of their hidden index finger after their elbow muscles had been conditioned with a flexion or extension contraction, or after a series of large passive elbow movements. After conditioning contractions, participants made small errors (∼2°) in perceived index finger position in the direction of elbow flexion, regardless of the contraction type. There was little to no effect of either contraction type on drift in perceived index-finger position in our test. Our results support the view that high-level proprioceptive judgements of hand position can be minimally affected by the effects of muscle thixotropy. Thus, we suggest that muscle spindle signals do not dominate the central, cross-modal transformations of sensory information that are required for high-level proprioceptive judgements.