Introducing the reflex probability maps in the quantification of nociceptive withdrawal reflex receptive fields in humans

The aim of the present study was to improve the assessment of reflex receptive fields (RRF) in humans, using reflex sensitivity and reflex probability maps. Repeated electrical stimulation was applied to elicit the nociceptive withdrawal reflex (NWR) in fifteen healthy volunteers using two stimulation paradigms: fixed (FSI) and adjusted (ASI) stimulation intensities. Stimulation was applied on sixteen sites in the foot sole, and pain intensity ratings and EMG responses were recorded. RRF sensitivity and probability maps were derived, and RRF areas were calculated. During FSI, the stimulation intensities were constant and the pain ratings dropped significantly (p < 0.01). In contrast, during ASI the pain ratings were stable, but there was a significant increase in the stimulation intensities (p < 0.01). None of the paradigms altered significantly the RRF areas, but the FSI paradigm had lower estimation error (p < 0.01). In all cases, the estimation error remained under 10% and 5% after five and ten repetitions, respectively. The 2nd stimulus in the train consistently rendered larger and more reliable RRF areas than the 1st stimulus. The present analysis can be useful in order to identify the most adequate stimulation parameters and quantification variables for RRF assessment in experimental and clinical pain research.