The influence of attentional factors on short- and long-latency jaw reflexes in man.

Reflexes evoked by applying non-painful taps to an incisor tooth were recorded bilaterally from the jaw-closing masseter and temporal muscles of 21 humans. A series of inhibitory, excitatory, inhibitory and excitatory waves (the Q, R, S and T waves) appeared in full-wave rectified and averaged post-stimulus electromyograms. These reflex responses were affected by the participants' levels of attention. When they undertook mental exercises in the form of arithmetic calculations, increases in electro-myographic activity were found around the transitions between the Q and R and the S and T waves. These increases involved principally a shortening of the inhibitory Q and S waves. There was no significant difference between the occurrence of these effects in the QR and ST segments. However, the effects were seen more commonly when the reflexes were evoked by hard (7.4 mN.s) as opposed to soft (3.4 mN.s) taps. It is concluded that, in man, attentional factors can modulate both short- and long-latency jaw reflexes, particularly when these are evoked by higher-threshold afferent nerves.