The role of the tactile-pressure afferents in the habituation phenomenon of trigemino-facial reflex.

In ten healthy subjects the trigemino-facial reflex was elicited by double shock (delta t = 0.5-1 s). The first stimulus was subliminal for pain and the second supramaximal for it. The percentage integral values, with respect to single shock, of the direct and consensual conditioned responses obtained in all subjects were averaged and compared with those conditioned by supramaximal stimuli for pain fibres. Significant differences between subliminal and supramaximal conditioning stimuli for pain fibres is revealed for only consensual R3 with delta t = 1 s: greater inhibition with painful stimuli. There were no significant differences in any of the other values. Our results show that a subliminal conditioning stimulus for pain threshold can inhibit the late responses to the test stimulus, both for R2 and especially for R3, suggesting that this mechanism can contribute to the trigemino-facial reflex habituation phenomenon.