Afferent Contributions to Digit Force Coupling and Force Level Variation During Performance of Non-lift Pinch

Abstract

Afferent contributions to the coordination of thumb and index finger forces during non-lift pinch were studied using an anesthetization case study design. Two subjects, one performing with and without digital anesthetization and one with intact sensation, produced dynamic pinch forces against a stable object, with and without visual feedback. Error corrections were less frequent post-anesthetization, and the cross correlation between digit forces was lower when sensation was removed. However, this decrease in cross correlation between digit forces seemed to reflect a loss in the magnitude of tightly coupled error corrections when sensation was removed, rather than more frequent deviations of force magnitude between the digit forces. Force-time output without visual feedback lacked these error corrections, and the correlation between digit forces remained high, irrespective of sensory status. Additionally, with vision occluded, the time rate of force change did not vary in a gradual manner as would be expected from a neural representation of a sinusoidal target, but was instead marked by sudden abrupt reversals of force rate of change, invariant of somatosensory status. The coupling of digit forces and rates of force change during non-lift pinch appear to be controlled primarily with feedforward mechanisms, where the lack of proprioceptive feedback does not seem to disrupt this coupling.

We wish to thank Dr. Stephan Zachary, DO, MS, Department of Orthopedics and Rehabilitation, University of Wisconsin, for providing the digital anesthetization and clinical testing of post-injection anesthesia; JoAnne Lazarus, PhD, University of Wisconsin-Madison, for her mentoring; and Rachel Seidler, PhD, University of Michigan, for her review of the error correction data and her discussion related to gain of display of the force signal. This research was partially supported by a Virginia Horne Henry Wisconsin Distinguished Graduate Fellowship awarded to the first author.