Gating of sensation and evoked potentials following foot stimulation during human gait

To investigate how gait influences the perceived intensity of cutaneous input from the skin of the foot, the tibial or sural nerves were stimulated at the ankle during walking or running on a treadmill. As compared to standing, the detection threshold for these stimuli was raised by more than 30% during the locomotion tasks. During walking, there was a phase-dependent modulation in perceived intensity of suprathreshold stimuli (1.5, 2, or 2.5 x PT). Stimuli given just prior to footfall were perceived as significantly above average (Wilcoxon signed-rank test). In contrast there was a significant phasic decrease in sensitivity for shocks delivered immediately after ipsi- and contralateral footfall. The amplitude of somatosensory evoked potentials (P50-N80 complex), simultaneously evoked from pulse trains to the sural nerve and recorded 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 potentials after touchdown may be due to occlusion or masking by concomitant 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 centrally generated facilitation of sensory transmission of signals in anticipation of foot-placing.