Voluntary Responses to Feedback Signals in Jaw Reflex Studies

Objective: In controlled studies of inhibitory jaw reflexes in human beings, visual feedback is employed to help the participants maintain a steady background activity in the muscles being studied. This feedback often consists of a processed EMG from one of muscles. However it is possible that a fall in the feedback signal due to an inhibitory reflex, might lead to a voluntary increase in EMG activity, which could be wrongly interpreted as a second (excitatory) reflex. The purpose of this study was to investigate this possibility.

Methods: Masseter EMG was recorded in 10 volunteers. These participants were provided with visual feedback of this EMG after it had been rectified and integrated (time-constant: 500ms). The participants were asked to use the feedback to maintain activity in the muscle at approximately 12.5% of maximum. While this was being done, the EMG (and hence the feedback signal) was occasionally 'muted' for 50ms to imitate the profound inhibitory response evoked during reflex studies. The EMGs were processed by rectification and averaging, and any deviations in the 800ms following the muted period were analysed objectively with respect to the EMG in a 300ms pre-mute period.

Results: Although there were some increases in the post-mute EMG, which could be adjudged to be significantly greater than deviations in activity in the pre-mute period, these were small and in no way comparable to the large increases in activity which are often seen following inhibitory responses in reflex studies. Furthermore, these small increases in the present study were found most consistently more than 400ms after the mute, which is much later than the timing of post-inhibitory excitations in reflex studies.

Conclusion: Voluntary responses to feedback signals are unlikely to contribute to any great extent to the EMG fluctuations which often follow inhibitory jaw reflexes.