Characterizing The Multimodal Sympathetic Nervous System Startle Response

The human startle reflex has primarily been characterized by its repeatable, coordinated, and temporally patterned somatomotor responses. In this study, we assessed whether startle-evoked sympathetic nervous system (SNS) responses might also constitute a repeatable, coordinated, and temporally structured output. Using a noninvasive tactile startle stimulus, we simultaneously recorded startle-evoked electrodermal activity, photoplethysmography-derived blood volume indices, heart rate, blood pressure, stroke volume, and cardiac output in healthy participants. Our results demonstrate that startle elicits a reproducible and patterned constellation of SNS responses - a multimodal SNS startle signature - with conserved temporal relationships across effector systems. The SNS startle signature was composed of robust bilateral peripheral responses, including palmar sweating, cutaneous vasoconstriction, and biphasic cutaneous venous-capillary blood volume changes, in addition to more mild central hemodynamic changes. In contrast to previous startle reflex studies, there was no influence of biological sex or cardiac-cycle gating on responses. Lastly, the SNS startle signature exhibited features of a potentially attractive diagnostic, associated with robust responder discrimination and high response reliability across repeated trials. Overall, these findings fill a critical knowledge gap and also suggest the potential utility of this multimodal signature for assessing autonomic dysfunction.