Abstract
Galvanic vestibular stimulation (GVS) modulates the firing of vestibular afferents (Goldberg et al. 1984) and consequently results in whole-body postural responses (Fitzpatrick & Day 2004). Similar to other sensory systems, the vestibular system habituates to repeated stimuli in both animals and humans. The habituation phenomenon is characterized by a response decrement to a repeating stimulus that is not associated with adaptation or fatigue. Although adaptation to GVS has been observed (St. George et al. 2011), there is limited evidence demonstrating habituation of postural responses associated with repetitive GVS. The purpose of this study was to determine whether the vestibular-evoked myogenic responses habituate in lower leg muscles. Participants stood upright with their eyes closed and head turned 90° to the right while surface electromyography was recorded from the left medial gastrocnemius and soleus. Participants were exposed to a predetermined order (balanced latin square) of sinusoidal GVS consisting of eight different frequencies (0.125–16.0Hz) presented over 6-min trials (single frequency per trial). Habituation of the muscle responses to the vestibular signal was quantified using frequency (coherence) and time (cumulant density) domain correlations. The GVS-induced myogenic responses decreased in both muscles throughout the trials. Myogenic responses elicited by 4 and 8Hz stimuli habituated to a greater extent than other frequencies tested. Our findings indicate that GVS-evoked myogenic responses habituate and that the habituation process is frequency-dependent. Considering that GVS provides a way to quantify vestibular control of muscles, future experiments should take into account the possibility of habituation while analyzing GVS-evoked responses.
