THE INHIBITION OF THE PUPILLARY LIGHT REFLEX BY THE THREAT OF AN ELECTRIC-SHOCK - A POTENTIAL LABORATORY MODEL OF HUMAN ANXIETY

It has been shown that the eye-blink response evoked by an abrupt loud white noise ('acoustic startle') is potentiated when the subjects ant icipate an aversive stimulus, e.g. an electric shock ('fear-potentiate d startle'). It has been proposed that this paradigm may be a useful l aboratory model of human anxiety. We examined whether the threat of an electric shock, as used in the fear-potentiated startle paradigm, wou ld affect the pupillary light reflex, in 12 healthy volunteers. Light stimuli (0.32 mW/cm(2), 200 msec) were generated by a light-emitting d iode, and pupil diameter was monitored by computerized binocular infra red television pupillometry in the dark. The light reflex was recorded during either the anticipation of a shock ('threat' blocks) or period s in which no shocks were anticipated ('safe' blocks). The shock consi sted of a single square wave current pulse (1.5 mA, 50 msec) applied t o the median nerve. At the end of each 'threat' or 'safe' block, subje cts rated their anxiety using visual analogue scales. Two-factor analy sis of variance (condition x block) showed that in the 'threat' condit ion there was a consistent increase in initial pupil diameter, a decre ase in light reflex amplitude and an increase in alertness and anxiety ratings. These effects were observable before the subjects received a ny shock (a single stimulation of the median nerve). These results sho w that the anticipation of an electric shock can modify not only the s tartle reflex response but also the pupillary light reflex, suggesting that the inhibition of the light reflex by threat may be another suit able laboratory model of human anxiety.