ORIGINAL ARTICLE Modulation of the startle reflex by heat pain: Does threat play a role? C. Horn-Hofmann, S. Lautenbacher Physiological Psychology, University of Bamberg, Germany Correspondence Claudia Horn-Hofmann E-mail: claudia.horn-hofmann@uni-bamberg.de Funding sources This study was supported by a research grant from the Deutsche Forschungsgemeinschaft (La 685/6-3). Conflicts of interest None declared. Accepted for publication 24 April 2014 doi:10.1002/ejp.539 Abstract Background: Previous studies have indicated that the startle reflex is potentiated by phasic, but not by tonic, heat pain, although the latter is seen as more strongly associated with emotional responses and more similar to clinical pain. The threat value of pain might be a decisive variable, which is not influenced alone by stimulus duration. Objective: This study aimed at comparing startle responses to tonic heat pain stimulation with varying degrees of threat. We hypothesized that the expectation of unpredictable temperature increases would evoke higher threat and thereby potentiate startle compared with the expectation of constant stimulation. Methods: Healthy, pain-free subjects (n = 40) underwent painful stimulation in two conditions (low/high threat) in balanced order. The only difference between the two conditions was that in the high-threat condition 50% of the trials were announced to include a short further noxious temperature increase at the end. Startle tones were presented prior to this temperature increase still in the phase of anticipation. Results: We observed startle potentiation in the high-threat compared with the low-threat condition, but only in those participants who took part first in the high-threat condition. Habituation could not account for these findings, as we detected no significant decline of startle responses in the course of both conditions. Conclusions: Our results suggest that subjective threat might indeed be decisive for the action of pain on startle; the threat level appears not only influenced by actual expectations but also by previous experiences with pain as threatening or not. 1. Introduction The experience of pain is often functionally associated with actual or impending injury. For this reason, pain is thought to be a particularly salient indicator for threat to our body′s integrity. Threatening stimuli acti- vate the motivational defence system (Lang et al., 1998; Bradley et al., 1999, 2005), which corresponds to a neural subcortical circuit organized around the amygdala preparing the organism to engage in protec- tive behaviour (Lang et al., 2000). According to the view of motivational priming, acti- vation of the motivational defence system can be mea- sured by recording defensive reflexes, as these reflexes should be enhanced in contexts when the defence system is activated (Lang et al., 1997). This assump- tion has been corroborated by research using the eye- blink component of the startle reflex (Lang, 1995). It is consistently shown that fear-evoking stimuli potenti- ate the magnitude of the blink reflex (Lang et al., 1990; Grillon et al., 1991; Grillon and Davis, 1997; Bradley et al., 1999; Grillon and Baas, 2003). However, studies investigating startle potentiation by pain have yet yielded inconclusive results. We found no potentiation of startle amplitude by tonic heat pain in three consecutive experiments, regardless © 2014 European Pain Federation - EFIC ® Eur J Pain •• (2014) ••–•• 1