A Robust Approach to Quantify Nocifensive Blink Reflex Responsiveness

The modulation of the hand-blink reflex (HBR), a prototypical nocifensive response, is increasingly used to investigate defensive behaviour, related to the notion of peripersonal space. However, HBR responsiveness is highly variable across participants. This variability has led researchers to use several seemingly arbitrary criteria to determine whether a subject should be included as a participant in a study. But are these criteria justified? Can better and more rigorous criteria be formulated? Does the traditional division into responders and non-responders reflect a practical decision to exclude participants with very low signal-to-noise ratio, or does it reflect two distinct biological categories? Here, we addressed these issues by systematically varying a set of parameters, which together form an objective and quantifiable criterion of HBR responsiveness. We describe classification criteria for HBR responsiveness that were both reliable and consistent with previous studies. We also found no evidence for a clear-cut biological distinction between HBR responders and non-responders. We recommend to (1) no longer preliminarily screen subjects, simply collecting data on all subjects, and (2), after collecting the data, only include subjects identified as blinkers using the following criteria: The mean of the rectified HBR must exceed 2.5 SD of the baseline EMG in 40% or more of trials in the hand-near condition. We formulate rigorous inclusion criteria for HBR studies, which can be adapted for use on other neurophysiological responses in health and disease.