Modulation of jaw reflexes by remote noxious stimulation and mental state: possible association with psychological measurements of mental stress and occupation.

Combined electrophysiological and psychophysical experiments were performed on 15 human subjects to investigate the possible effects of perceived stress or mental occupation on jaw reflexes. Electromyographic recordings were made from the masseter and anterior temporalis muscles, of the series of excitatory and inhibitory reflexes evoked by tapping on an upper incisor tooth. The reflexes were modified by application of painful cold (3 degrees C) stimuli to the subject's hand (remote noxious stimulation) or by the subject undertaking mental exercises (the 17 times table). The resulting changes in the reflexes usually involved transient increases in EMG activity around the interfaces between successive inhibitory and excitatory responses. Both the remote noxious stimuli and the mental exercises usually produced increases in both stress and mental occupation as assessed using visual analogue scales. However, correlations between these psychological effects and the effects on the reflexes were generally weak or absent. We conclude that the modulation of jaw reflexes by remote noxious stimuli or mental activity is not likely to be dependent on an individual's conscious awareness of a change in mental state. On the other hand, data from a related study suggest that the effects on the reflex may be more closely related to the autonomic responses to stress.