The acoustic startle reflex has proved to be an excellent system for s
tudying habituation in mammals. In animal studies, startle habituation
has been found to be sensitive to various pharmacological manipulatio
ns. The present experiments were designed to determine whether caffein
e (4 mg/kg) modified startle habituation in low and high caffeine user
s. Human eyeblink responses were measured in a startle habituation par
adigm in which 30 trials of 85-dB broadband noise stimuli with a durat
ion of 50 msec and a rise time of .1 msec were presented. Caffeine del
ayed the habituation of startle amplitude in both low and high users a
nd produced significant dishabituation in low users. These findings in
dicate that caffeine disrupts early sensory filtering.