The conceptualisation of the nature of fear and anxiety from the behavioural neuroscientific approach has seen considerable advancement in recent years. This advancement has been significantly aided by work conducted with nonhuman animals using fear-potentiated startle as a measure of fear during aversive Pavlovian conditioning (Fanselow & Poulos, 2005). Research utilising this methodology has led to the delineation of the neural circuitry involved in fear learning in nonhuman animals (Davis, Walker, & Lee, 1999; Fellous & LeDoux, 2005). Studies conducted primarily with rodents have shown that the pathway mediating the startle response intersects with the fear pathways, validating the use of the reflex as a measure of fear (Davis & Lee, 1998).
Startle has subsequently been utilised to assess the acquisition and extinction of fear in humans. The validity of startle as a measure of emotion during the perception of emotive stimuli has been well supported (Bradley, Cuthbert, & Lang, 1999). However, the validity of startle as a measure of fear in anticipation of aversive stimuli has not yet been conclusively validated. This is because startle is not only sensitive to the valence of a stimulus, but also to its attentional or arousal properties (Dawson, Schell, & Böhmelt, 1999).
Previously, the modality of the attended-to stimulus has been used as a means to distinguish attentional and emotional influences on the startle response as it was thought that attentional startle facilitation depended on a match between the modality of the startle eliciting stimulus and attended-to stimulus (Anthony & Graham, 1985; Putnam, 1990). However, recent studies have repeatedly shown that startle can be facilitated during an attention-grabbing stimulus, irrespective of the match between the modality of that stimulus and the startle probe (Böhmelt, Schell, & Dawson, 1999; Lipp, Neumann, & McHugh, 2003; Lipp, Neumann, Pretorius, & McHugh, 2003; Lipp, Siddle, & Dali, 1997, 1998, 2000a, 2000b; Neumann, Lipp, & McHugh, 2004). Therefore, in conditioning studies, facilitated startle during the cue for an aversive stimulus compared to a stimulus presented alone, could reflect either aversive emotional processes or increased attention and arousal.
The main aim of the current research project was to clarify the previously ambiguous findings regarding the nature of the processes reflected in startle modulation in anticipation of an aversive stimulus. The aim was to introduce new control procedures, as well as using existing controls, to separate the influences of attention/arousal and affect on the startle response during human fear conditioning. (Note that although attention and arousal are considered distinct processes, they become difficult to differentiate when it comes to startle, so the term "attention/arousal" is used at such times.) Five studies were undertaken in order to achieve this goal.
Tactile stimuli were introduced in order to separate the influence of attention and emotion on startle. Findings from studies employing pulsing tactile stimuli show that startle elicited during such stimuli is modulated by the affective significance of the stimulus, but not by its attentional properties (Lipp et al., 1998; Lipp & Hardwick, 2003; Lipp, Neumann, & Pretorius et al., 2003). It was suggested that this may be because of the pulsing nature of the stimulus which may result in prepulse inhibition and subsequent reduction in attentional facilitation of startle (Lipp, Neumann, & Pretorius et al., 2003). ..............................