Facilitation and Habituation of Cortical and Subcortical Control of Standing Balance Following Repeated Exposure to a Height-related Postural Threat.

Threats to stability elicit context-specific changes in balance control; however, the underlying neural mechanisms are not fully understood. Previous work has speculated that a shift toward greater supraspinal control may contribute to threat-related balance changes. This study investigated how neural correlates of cortical and subcortical control of balance were affected by initial and repeated exposure to a height-related postural threat. Corticomuscular coherence (CMC) between EEG recorded over the sensorimotor cortex and EMG recorded from the soleus (SOL) provided an estimate of cortical control, while intermuscular coherence (IMC) between bilateral SOL provided estimates of both cortical and subcortical control. These outcomes, along with measures of psychological and arousal state and standing balance control, were examined in 28 healthy young adults during a series of 90-s quiet standing trials completed at LOW (0.8 m above ground; away from edge) and HIGH (3.2 m above ground, at edge) threat conditions. Initial exposure to threat significantly increased gamma-band CMC (31-40 Hz) and IMC at frequencies thought to be mediated by cortical (21-40 Hz) and subcortical (5-20 Hz) substrates. Following repeated threat exposure, only estimates of cortical control (gamma CMC and 21-40 Hz IMC) demonstrated significant habituation. Estimates of cortical control changed in parallel with high-frequency centre of pressure power (>0.5 Hz) and plantar-dorsiflexor coactivation, but not other threat-related balance changes which did not habituate. These results support the hypothesis that postural threat induces a shift toward more supraspinal control of balance, and suggests this altered neural control may contribute to specific threat-related balance changes.