Comparison of the effects of diazepam on the fear-potentiated startle reflex and the fear-inhibited light reflex in man

It has been shown previously that the amplitude of the acoustic startle ref lex is enhanced, and the amplitude of the light reflex reduced, when subjec ts anticipate an aversive event, compared to periods when subjects are rest ing ('fear-potentiated startle reflex' and 'fear-inhibited light reflex'). We examined whether the anxiolytic diazepam would reverse the effects of th reat on the startle and pupillary reflexes. Twelve male volunteers particip ated in three weekly sessions in which they received oral treatment with pl acebo, diazepam 5 mg and diazepam 10 mg, according to a balanced crossover double-blind design. One hour after ingestion of the treatments, miotic res ponses to light pulses and electromyographic responses of the orbicularis o culi muscle to sound pulses were elicited during alternating periods in whi ch the threat of an electric shock (electrodes attached to the subject's wr ist) was present (THREAT) and absent (SAFE). The THREAT condition was assoc iated with a significant increase in the amplitude of the electromyographic (EMG) response, a significant reduction of the miotic response amplitude, and an increase in self-rated anxiety. Diazepam attenuated all these effect s of THREAT. Diazepam did not affect the amplitude of the miotic response u nder the SAFE condition, but did suppress the EMG response under this condi tion. These results confirm the validity of the fear-potentiated startle re flex and fear-inhibited light reflex as laboratory models of human anxiety, and reveal some differences between the effects of diazepam on the two ref lexes.