Not just noise: Individual differences in general startle reactivity predict startle reactivity to uncertain and certain threat.
Bradford DE, Kaye JT, Curtin JJ (2014). Psychophysiology, 51, 407-411. PMC3984356. PDF

What can the blink of an eye tell us about someone’s typical response to stressors? Our research team has long been interested in how the startle response can provide a window into people’s emotional traits and states. If you have ever jumped out of your seat to an unexpected loud noise in a horror movie, then you know the startle response. In our laboratory we measure this reflexive response in a more precise manner. By recording electrical activity in the muscle below the eye we can quantify how strongly research participants blink when they are surprised by a loud noise. When participants arrive at our lab, we routinely evaluate how strongly they blink before the start of the main experiment. It turns out that people differ widely in the strength of this 'general startle reactivity'.

Many researchers consider these individual differences in general startle reactivity to be unimportant (what scientists call ‘noise’ in the experiment). However, we were interested in whether general startle reactivity might be related to people’s stress reactivity in interesting ways. After evaluating their general startle reactivity, all participants completed a stressful experimental task. Participants experienced a series of stressful events (electric shock to their fingertips) that were predictable (certain threat) in one condition and unpredictable (uncertain threat) in another condition. We also included a third control condition where they experienced no stressor. We used several different tasks (i.e., NPU, Probability, Duration tasks) that manipulated how the stressor was unpredictable; across these tasks, participants did not know either if, when, or how likely the shock was to occur at any moment in the unpredictable condition. Overall, general startle reactivity was more strongly related to peoples' startle response during two threat conditions compared to when they were in the safe (no threat) condition. Furthermore, general startle reactivity was more strongly related to startle responses during uncertain than certain threat. These results show that general startle reactivity is 'not just noise' but in fact may be related to underlying traits or dispositions in how people respond to different types of stressors. In addition to the theoretical contributions, this paper has important methodological implications and suggests that researchers may increase their power and precision of their analyses of the startle response by using general startle reactivity as a control variable (covariate). This study adds to a growing body of research from our and other laboratories that suggests that general startle reactivity may contribute as a neurobiological index of fear circuitry from the emerging National Institute of Mental Health (NIMH) Research Domain Criteria perspective.

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