Effects of threat of shock, shock electrode placement and darkness on startle.

Fear can be elicited by physically-presented explicit threat stimuli or by more static contextual stimuli that are not an immediate source of danger. Research in both humans and animals suggest that fear produced by these two types of stimuli represents separate processes mediated by different brain structures. The present study used the startle reflex methodology to examine affective responses elicited by an explicit threat cue signalling a period of shock anticipation and by two types of contextual stimuli; darkness and attaching the shock electrodes. As expected, shock anticipation potentiated startle (fear-potentiated startle). Startle was also facilitated by darkness and by the placement of shock electrodes. Further, darkness increased fear-potentiated startle to an explicit threat cue, but did not affect the facilitation of startle produced by attaching the shock electrodes. It is suggested that affective responses to contextual stimuli should be considered when investigating both normal and pathological fear.