To blink or not to blink: inhibition and facilitation of reflex blinks

A reflex blink typically inhibits subsequent blinks. In this study, we investigated whether the nature and time course of this inhibition vary when different combinations of blink-evoking stimuli are used. We used the paired stimulus paradigm, in which two blink-evoking stimuli-a conditioning stimulus followed by a test stimulus-are presented with a variety of interstimulus intervals, to examine the interactions between blinks evoked by trigeminal and acoustic stimuli in rats and humans. In addition, we studied the effect of a blink-evoking trigeminal stimulus on subsequent gaze-evoked blinks in humans. The results revealed that long-lasting inhibition occurred when the conditioning and test stimuli were within the same modality. A shorter period of inhibition followed by facilitation occurred when the stimuli were in different modalities. The data demonstrate that a blink-evoking stimulus initiates a lengthy period of inhibition in its own sensory pathway and a shorter period of inhibition in the reticular formation and/or in blink motoneurons. In addition, the results show that the blink-evoking stimulus also initiates a facilitatory process. Thus, the magnitude of a blink reflects a balance between inhibitory and facilitatory processes.