Ali Doroodchi, Sean Lai, Sue Peters
The brain contributes to maintaining balance both while standing and during step initiation, where cortical preparatory activity and sensory modulation may be tuned to the task goal. Standing requires maintaining postural equilibrium, whereas stepping involves propelling the body toward a target while preserving stability. These demands change with the intended step direction, suggesting that cortical processing may be flexibly adjusted according to directional goals. Foot-sole cutaneous receptors are relevant to postural control, as they signal load distribution that supports upright posture and step initiation. This project aimed to investigate how sensory modulation, and preparatory cortical processes differ across stepping tasks with differing directional goals. Quiet standing, straight stepping, and diagonal stepping tasks were performed by 31 healthy participants, while EEG was recorded. Standing trials were conducted only with electrical stimulation, whereas stepping trials were performed both with and without stimulation (2 Hz at 125% perceptual threshold) delivered to the left (stance) foot sole to elicit somatosensory evoked potentials (SEPs). Trials without stimulation allowed examination of cortical preparatory activity. eLORETA source localization was applied across all conditions to identify cortical generators. SEP amplitudes were larger during both stepping tasks compared to standing, and diagonal stepping SEPs were larger compared to straight stepping. In non-stimulation trials, frontal, central, and parietal regions showed greater preparatory activity during diagonal compared with straight stepping, particularly in early preparation stages. These findings suggest that somatosensory processing and preparatory cortical activity are flexibly tuned to the directional goal of the task, consistent with optimal feedback control principles.