The inhibition of the pupillary light reflex by the threat of an electric shock: a potential laboratory model of human anxiety

Abstract

It has been shown that the eye-blink response evoked by an abrupt loud white noise ('acoustic startle') is potentiated when the subjects anticipate an aversive stimulus, e.g. an electric shock ('fear-potentiated startle'). It has been proposed that this paradigm may be a useful laboratory model of human anxiety. We examined whether the threat of an electric shock, as used in the fear-potentiated startle paradigm, would affect the pupillary light reflex, in 12 healthy volunteers. Light stimuli (0.32 mW/cm², 200 msec) were generated by a light-emitting diode, and pupil diameter was monitored by computerized binocular infrared television pupillometry in the dark. The light reflex was recorded during either the anticipation of a shock ('threat' blocks) or periods in which no shocks were anticipated ('safe' blocks). The shock consisted of a single square wave current pulse (1.5 mA, 50 msec) applied to the median nerve. At the end of each 'threat' or 'safe' block, subjects rated their anxiety using visual analogue scales. Two-factor analysis of variance (condition x block) showed that in the 'threat' condition there was a consistent increase in initial pupil diameter, a decrease in light reflex amplitude and an increase in alertness and anxiety ratings. These effects were observable before the subjects received any shock (a single stimulation of the median nerve). These results show that the anticipation of an electric shock can modify not only the startle reflex response but also the pupillary light reflex, suggesting that the inhibition of the light reflex by threat may be another suitable laboratory model of human anxiety.