Foot Sole Cutaneous Somatosensory Modulation Based on Balance Demands

Postural complexity may shape how the nervous system processes plantar cutaneous input. We tested whether somatosensory evoked potentials (SEPs) elicited by foot-sole stimulation scale with balance demands, hypothesizing larger responses for more complex tasks. Thirty-one healthy adults performed standing, straight-step, and diagonal-step conditions while receiving brief electrical stimulation to the stance foot sole; SEPs (P50, N90, peak-to-peak) were analyzed at Cz using pooled and order-specific approaches. In the pooled analysis, peak-to-peak SEP amplitude was greater for both stepping conditions than standing (Standing vs. Straight, P = 0.041; Standing vs. Diagonal, P = 0.026). Order-specific analysis showed an early amplification: the first SEP (0.5 s after the warning cue) was larger for diagonal than straight stepping (peak-to-peak, P = 0.009; N90, P = 0.027). Source localization at N90 revealed greater activation during stepping than standing in paracentral gyrus & sulcus, inferior parietal angular gyrus, and superior parietal gyrus, consistent with enhanced sensorimotor processing under higher postural demands. Moreover, right paracentral gyrus & sulcus activity was higher for diagonal vs. straight stepping for the first and fourth SEPs. Together, these findings indicate that increasing balance demands up-weight plantar afferent processing and recruit contralateral sensorimotor/parietal regions, particularly early in preparation, supporting the view that cortical sensory gain is tuned to postural complexity.