Vestibular Modulation of the Abductor Hallucis and the Abductor Digiti Minimi Muscles in Response to Changes in Head Position, Visual Cues, and Cognitive Demand
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Maintaining standing balance involves processing of vestibular, visual, and somatosensory information to produce dynamic motor responses. Bilateral electrical vestibular stimulation (EVS) delivered through the mastoid processes can be used to explore the vestibular system. The purpose of this experiment was to determine if intrinsic foot muscles are modulated by vestibular activity and to elucidate any changes in the association between the vestibular stimulation and electromyography (EMG) responses in response to changes in head position, visual cues, and cognitive demand. Indwelling EMG of the abductor hallucis (AH) and abductor digiti minimi (ADM) were sampled while EVS was administered to quietly standing participants. The relationships between the EVS input and the muscle activity (output) were characterized in both the time and the frequency domains. When the head orientation was changed from left to right, the biphasic vestibular response in the time domain was inverted. In conditions including visual cues and increased cognitive demand, the vestibular responses demonstrated reduced and increased amplitudes of the coherence function, respectively. These findings indicate that the vestibular system modulates activity in the ADM and AH during quiet standing balance tasks.