Motor Control and Self-Regulatory Fatigue Following Facial Nerve Paralysis

Abstract

This research project addressed the question of why facial nerve paralysis patients experience fatigue during facial expression exercises and when using their face in social situations. It was proposed that attempts at consciously controlling the movement of the usually automatically controlled facial muscles deplete cognitive resources, and result in a task-specific mental fatigue comparable to the depletion observed following self-regulation efforts. A questionnaire study showed that facial exercises were as fatiguing as performing moderate gardening or heavy housework; and for patients, use of the face in a social setting was as fatiguing as walking up a steep hill for 1km, supporting the resource depletion view. Conscious control of the face for facial nerve paralysis patients may also be fatiguing due to the fine movements needed to perform facial expressions in social interactions, so an apparatus for testing sensitivity of proprioception of the orofacial muscles was developed, resulting in the first research on orofacial muscle proprioception in comparison to the jaw muscles. By using the same psychophysical task it was possible to accurately measure the proprioceptive ability of the pucker movement of the lips and for the closing movement of the jaw. The acuity of the orofacial muscles, a structure devoid of muscle spindles, was significantly better than for the jaw, an area with high proprioceptor (muscle spindle) density. Further, there was a significant correlation between the proprioceptive sensitivity scores of the lips and jaw, suggesting that proprioceptive sensitivity arises from extensive coordinated use of the lips and jaw for expression, speech and mastication. The high degree of proprioceptive ability demonstrated by the lips, as well as the correlation with the jaw highlights the unique nature of the facial muscles of expression; and implies that the face should be treated as a separate entity in terms of exercise prescription.