Threat vs control: Potentiation of the trigeminal blink reflex by threat proximity is overruled by self-stimulation.

The magnitude of the defensive blink reflex is modulated by continuous assessment of its protective value. Here, we studied whether the trigeminal blink reflex (TBR) is modulated by a potentially offensive object close to the face, and, if so, whether self-stimulation or observation of the act of stimulus triggering counteracts such modulation. In all, 26 healthy volunteers participated in various experimental conditions. At baseline, an experimenter triggered supraorbital nerve stimuli remotely, unseen by the participants; in experimental conditions, the experimenter held a stimulation probe close to the participant's face but triggered the stimuli either remotely, "surprising" participants (S1 ), or directly on the probe, observed by participants (S2 ). In other conditions, participants triggered stimuli themselves on the probe held next to their body (S3 ) or held in front of their face (S4 ). The latter condition was repeated similarly, but pressing the button only randomly generated electrical stimuli (S5, "Russian roulette"). The size of the R2 component of the TBR (TBR-R2) was the main outcome measure. Compared to baseline, TBR-R2 area was significantly larger in S1 when the "threatening" probe was close to the face and the participant had no control over stimulation. Conversely, TBR-R2 was suppressed when participants either saw the action of triggering, thus being aware (S2 ), or had full initiative over stimulation (S3 , S4 ). Random self-generated stimuli (S5 ) inhibited TBR-R2, but to a lesser extent than S3 and S4. Perceived threat close to the face facilitates TBR-R2, but knowledge about impending stimulation or self-agency overrules this effect.