SENSITIVITY OF THE FEAR-INHIBITED LIGHT REFLEX TO DIAZEPAM

We have shown previously that pupil diameter increases and the amplitu de of the pupillary light reflex is reduced when subjects are under th reat of an aversive event (electric shock), and that light reflex ampl itude correlates negatively with subjective anxiety. We have suggested that the ''fear-inhibited light reflex'' paradigm could be used as a laboratory model of human anxiety. In the present study, we examined w hether two doses (5 mg and 10 mg) of the anxiolytic drug diazepam woul d antagonize the effects of threat on the pupillary light reflex. Twel ve healthy male volunteers participated in three weekly sessions, each associated with one of three treatments (diazepam 5 mg or 10 mg or pl acebo) in a double-blind, balanced, crossover design. The light reflex was recorded during either the anticipation of a shock (''threat'' bl ocks) or periods in which no shocks were anticipated (''safe'' blocks) . At the end of each ''threat'' or ''safe'' block, subjects rated thei r anxiety using visual analogue scales. Two-factor ANOVA (treatment x condition) showed that diazepam treatment antagonized the effect of th reat on light reflex amplitude in a dose-dependent manner but it did n ot affect the threat-induced increase in pupil diameter. Diazepam had no effect on the pupillary light reflex in the ''safe'' condition. Dia zepam also reduced subjective anxiety and alertness in the threat cond ition. These results show the sensitivity of the threat-induced reduct ion of light reflex amplitude to anxiolytic drugs, and provide further evidence for the utility of the fear-inhibited light reflex paradigm as a laboratory model of human anxiety.