This study investigated the size of, and relationship between, differe
nt modulatory effects of aversive stimulation on the acoustic startle
reflex. This reflex is potentiated by shock exposure and associative s
hock conditioning (in animals and human volunteers) and unpleasant pic
tures (tin human volunteers). In this study, dramatic sensitization of
the probe-startle response was observed after shock exposure but not
after a control task. Magnitude of sensitization was significantly lar
ger than associative shock conditioning and picture modulation effects
(also significant). Sensitization and conditioning scores showed mode
st, significant correlations with one another but not with picture mod
ulation scores, consistent with animal data showing that partially ove
rlapping brain mechanisms (i.e., amygdaloid-reticular projections) med
iate these effects. The present results also indicate that sensitizati
on of startle in human volunteers is a relatively more robust defensiv
e response to aversive stimulation.