The sensory input, not the motor output, defines blink reflex conditioning

Objective: Blink reflexes following supraorbital nerve (SON) stimulation are typically modulated by conditioning stimuli (CS) to the index finger (D2) (low-intensity, prepulse inhibition paradigm) or SON (same intensity, paired-pulse paradigm). We aimed to disentangle whether CS-intensity or CS-induced motor responses define blink reflex modulation. Methods: In 35 subjects, test SON stimuli (8 times sensory threshold, 8 x ST) were applied either alone or following CS. In experiment 1, CS were delivered to D2 with low (2 x ST) or high intensity (inducing a somatosensory blink reflex). In experiment 2, CS were applied to SON with low (<2 x ST) or test intensity. Test blink reflex size was correlated to CS-intensity and to CS-induced motor response size. Relative strength of their influence was determined in regression analyses. Results: Test blink reflex size showed higher inverse correlation to CS-intensity than to CS-induced motor response size for both CS delivered to D2 or SON. Regression analyses confirmed a significantly higher relative strength of CS-intensity than of CS-induced motor response size. Conclusions: The sensory input of CS, rather than CS-induced motor output, defines subsequent blink reflex modulation.