GATING OF SENSATION AND EVOKED-POTENTIALS FOLLOWING FOOT STIMULATION DURING HUMAN GAIT

To investigate how gait influences the perceived intensity of cutaneou s input from the skin of the foot, the tibial or sural nerves were sti mulated at the ankle during walking or running on a treadmill. As comp ared to standing, the detection threshold for these stimuli was raised by more than 30% during the locomotion tasks. During walking, there w as a phase-dependent modulation in perceived intensity of suprathresho ld stimuli (1.5, 2, or 2.5xPT). Stimuli given just prior to footfall w ere perceived as significantly above average (Wilcoxon signed-rank tes t). In contrast there was a significant phasic decrease in sensitivity for shocks delivered immediately after ipsi- and contralateral footfa ll. The amplitude of somatosensory evoked potentials (P50-N80 complex) , simultaneously evoked from pulse trains to the sural nerve and recor ded at scalp level, was, on average, 62% of the level during standing. During gait, the amplitude of this complex was significantly smaller just after footfall than the amplitude during late swing (MANOVA). It is suggested that the reduced sensation and the decreased evoked poten tials after touchdown may be due to occlusion or masking by concomitan t afferent input from the feet. On the other hand, the phasic increase in sensitivity at the end of swing is thought to result from a centra lly generated facilitation of sensory transmission of signals in antic ipation of foot-placing.