Fig. 1.

Series of posterior root-muscle (PRM) reflexes without and with periodic amplitude modulation elicited by epidural stimulation of the lumbosacral spinal cord. Ai, top: responses of hamstrings (Ham) to 2-Hz stimulation with unmodulated amplitudes. Bottom: periodic reflex modulation elicited at 22 Hz in the same muscle group (note different time scaling). ii: Same PRM reflexes as shown in i but presented stimulus triggered in enlarged timescale. n, Numbers of superimposed responses. Bi: examples illustrating the characteristic short buildup phase of periodic modulation patterns, here elicited at 16 Hz, established either with the first pair of stimuli applied (top) or after a few stimulus pulses (bottom). ii: Peak-to-peak amplitudes (black circles) of the consecutively elicited PRM reflexes shown in i. Arrows mark times of stimulus application. Data derived from subject 2, epidural electrode combination 0− 3+, 6 V in Ai, both traces, and Bi, bottom trace; and 0+ 3−, 3 V in Bi, top trace.