Somatosensory changes associated with motor skill learning

Trial-and-error motor adaptation has been linked to somatosensory plasticity and shifts in proprioception (limb position sense). The role of sensory processing in motor skill learning is less understood. Unlike adaptation, skill learning involves the acquisition of new movement patterns in the absence of perturbation, with performance limited by the speed-accuracy trade-off. We investigated somatosensory changes during motor skill learning at the behavioral and neurophysiological levels. Twenty-eight healthy young adults practiced a maze-tracing task, guiding a robotic manipulandum through an irregular two-dimensional track featuring several abrupt turns. Practice occurred on days 1 and 2. Skill was assessed before practice on day 1 and again on day 3, with learning indicated by a shift in the speed-accuracy function between these assessments. Proprioceptive function was quantified with a passive two-alternative forced-choice task. In a subset of 15 participants, we measured short-latency afferent inhibition (SAI) to index somatosensory projections to motor cortex. We found that motor practice enhanced the speed-accuracy skill function ($F_{4,108}$ = 32.15, P < 0.001) and was associated with improved proprioceptive sensitivity at retention ($t_{22}$ = 24.75, $P$ = 0.0031). Furthermore, SAI increased after training ($F_{1,14}$ = 5.41, $P$ = 0.036). Interestingly, individuals with larger increases in SAI, reflecting enhanced somatosensory afference to motor cortex, demonstrated larger improvements in motor skill learning. These findings suggest that SAI may be an important functional mechanism for some aspect of motor skill learning. Further research is needed to test what parameters (task complexity, practice time, etc.) are specifically linked to somatosensory function

A Tablet-Based Tool for Accurate Measurement of Hand Proprioception After Stroke

Background and Purpose: Proprioceptive deficits in the hand are common following stroke, but current clinical measurement techniques are too imprecise to detect subtle impairments or small changes. We developed a tablet-based tool to measure static hand proprioception using an adaptive staircase procedure. Methods: In 16 individuals with chronic stroke and age-matched controls, we quantified proprioception at the metacarpophalangeal joint of the index finger using 3 methods: the tablet task, a custom passive movement direction discrimination test (PMDD), and a manual assessment similar to the Fugl-Meyer (F-M) proprioception subsection. Results: The tablet-based measure and the PMDD both identified impaired proprioception in the affected hand relative to the unaffected hand (P = 0.024 and 0.028), and relative to the control group (P = 0.040 and 0.032), while manual assessment did not. The PMDD had a ceiling effect as movement excursions greater than 15^ were not biomechanically feasible. The tablet-based measure and the PMDD detected impaired proprioception in 56% to 75%, and the F-M in only 29%, of patients. PMDD and tablet-based measures were both correlated with primary tactile sensation, but not manual dexterity. Discussion and Conclusions: Both the tablet-based tool and the custom PMDD performed better than manual assessment. The PMDD may be useful when the deficit is mild or assessment of dynamic proprioception is desired. As the tablet-based measure does not have the ceiling effect that is associated with the PMDD, it may be useful with any level of proprioceptive impairment, and may be preferable if testing or clinician training time needs to be minimized, or pain or spasticity is present