Bidirectional modulation of the R2 blink reflex using a lower limb cuff-pressure heterotopic conditioning stimulus across repeated sessions.

The R2 blink reflex (R2BR) provides a neurophysiological marker of trigeminofacial somatosensory processing in the human brainstem. In the current study, we paired the R2BR with a heterotopic noxious conditioning stimulus (HNCS) applied via semi-automated cuff pressure algometry to the lower limb across repeated test sessions. Twenty-four healthy participants completed two identical sessions in which the R2BR stimulus-response function was assessed across 11 intensities (0.9-1.9x threshold) at baseline, during tonic cuff-pressure conditioning applied to the contralateral lower limb, and during post-conditioning recovery periods. Overall R2BR excitability was also quantified using an excitability index derived from the stimulus-response function. The HNCS produced bidirectional effects on R2BR excitability, causing both inhibition and facilitation of the reflex across repeated sessions. Post-conditioning R2BR responses returned to baseline levels. The baseline R2BR excitability index demonstrated fair-to-good test-retest reliability (ICC = 0.73), whereas reliability during conditioning was reduced to poor-to-fair (ICC = 0.57). These findings demonstrate that a HNCS can modulate R2BR responses in a bidirectional manner, which was observed across repeated test sessions. These results indicate that the combined use of a semi-automated lower limb cuff pressure HNCS with the R2BR may provide a complementary neurophysiological approach for assessing descending inhibitory and facilitatory influences over brainstem reflex pathways. PERSPECTIVE: The R2 blink reflex (R2BR) paired with a semi-automated cuff-pressure HNCS provides a neurophysiological approach for assessing bidirectional endogenous modulatory influences on brainstem reflex excitability.