Differential effect of visual and gustatory stimuli on experimental jaw muscle pain

Emotions form an important component in the expression and manifestation of pain. This study tested whether emotionally loaded visual and gustatory conditioning stimuli could influence experimental jaw muscle pain.

Thirty-one healthy subjects participated in this study. In two sessions, experimental jaw muscle pain was evoked by injection of hypertonic saline (HS) into the masseter muscle. In the first session, jaw muscle pain was conditioned by positive, negative or neutral pictures. In the next session, jaw muscle pain was conditioned by sweet, bitter or tasteless gelatine in the mouth. The subjects continuously rated pain intensity as well as mood and unpleasantness/pleasantness of the conditioning stimuli. As a control experiment, the same design was applied in eight subjects using painful thermal stimuli.

HS evoked moderate levels of pain in all subjects. During conditioning with negatively loaded pictures, HS-evoked pain was higher compared with neutral and positive pictures (p = 0.002). There was a significant correlation between negative emotional scores linked to the pictures and HS-evoked pain intensity (r = 0.404, p = 0.021). No effects on HS-evoked pain intensity were observed during positive pictures or during sweet or bitter gustatory stimuli. The latter may be due to failure of the gustatory stimuli to generate changes in emotional scores. No significant effects were observed on thermal pain sensitivity.

These results suggest that not all conditioning stimuli are potent modifiers of emotions, which seems to be a prerequisite for effects on muscle pain perception.