The fear-inhibited light reflex: importance of the anticipation of an aversive event.

RATIONALE It has been shown previously that the amplitude of the pupillary light reflex response decreases when subjects anticipate an aversive stimulus (i.e. electric shock), compared to periods when subjects are resting ('fear-inhibited light reflex'). OBJECTIVE To compare the effects of the anticipation of an electric shock (putative aversive event) and of an acoustic stimulus (putative neutral event) on the light reflex. METHODS Twelve healthy volunteers participated in a training session and an experimental session. Pupil diameter was monitored with infra-red binocular television pupillometry. The experimental session consisted of 14 blocks of 3 light stimuli. 'Relaxation' (no anticipation) and 'anticipation' (electrical or acoustic stimulus) blocks alternated. Mood and feelings were self-rated on visual analogue scales. RESULTS The anticipation of the electrical stimulus was associated with increases in initial pupil diameter and subjectively rated 'anxiety' and 'alertness', and a decrease in the amplitude of the pupillary light reflex response, whereas anticipation of the acoustic stimulus was associated with increases in initial pupil diameter and subjective 'alertness' only. CONCLUSIONS The increase in initial pupil diameter is related to the anticipation of any stimulus, whereas the decrease in the amplitude of the light reflex response is associated with the aversiveness of the anticipated stimulus.