The balance N1 attenuates but persists during self-initiated and repeated perturbations, even without active balance control

Standing balance requires rapid responses to postural disturbances, and the balance N1 has been proposed to reflect cortical involvement in corrective motor output. Our recent work shows, however, that the N1 persists even when active balance control is removed and corrective muscle activity is largely absent, suggesting it reflects sensory rather than motor processing. Because the N1 is also modulated by contextual factors such as postural threat, stability, and perturbation predictability, it may reflect uncertainty associated with perturbation-evoked sensory signals. We tested this in 11 participants (3 males and 8 females) using toes-up perturbations across three manipulations expected to alter sensorimotor uncertainty: perturbation timing through 15 unexpected perturbations before and after a block of self-initiated trials; repeated exposure through 60 self-initiated trials; and body-motion variability by comparing active balance with passive support and by imposing prerecorded whole-body motion during 60 additional self-initiated trials with passive support. Self-initiation reduced N1 amplitude by 21.3 ± 13.9% relative to unexpected perturbations, and this attenuation was equivalent across active and passive conditions. Habituation through repeated self-initiated perturbations produced a further 24.0 ± 17.7% decline, again independent of active balance. When perturbations were subsequently delivered unexpectedly, N1 amplitudes increased, confirming that attenuation was specific to the self-initiated context. Finally, imposing prerecorded whole-body motion during passive support did not change perturbation-evoked N1 amplitude, suggesting that body-motion variability does not drive the cortical response when active balance control is absent. These results indicate that the balance N1 is shaped by predictability and repetition, consistent with prediction error or surprise-based accounts of cortical responses to balance perturbations. Significance statement The balance N1, a cortical response to postural disturbances, has long been thought to reflect corrective motor output. We recently showed it instead reflects sensory processing, as it persists even without active balance control. Here we test whether this processing scales with uncertainty associated with perturbation-evoked sensory signals by manipulating perturbation timing through self-initiation, habituation through repeated exposure, and variability of ongoing whole-body motion. Self-initiation and repetition both attenuated N1, regardless of whether participants actively balanced or were passively supported. However, increasing whole-body motion variability did not modify the response, indicating that not all sources of uncertainty modulate the N1. These findings recast the balance N1 as tracking the sensory predictability of the perturbation, not balance-correcting motor output.