Human perception of self-motion and orientation during galvanic vestibular stimulation and physical motion
Fig 2
Providing an extension of the vestibular afferent transfer functions evoked by GVS to frequencies beneath 0.1 Hz via the essential works of Kwan et al. [29] and Forbes et al. [28].
a, Depiction of binaural bipolar GVS’s depolarizing and hyperpolarizing effect. The cathode side (negative terminal) increases irregular and regular firing rates (i.e., depolarization). Conversely, the anode side (positive terminal) decreases firing rates (i.e., hyperpolarization). b, Resultant firing rates following a 1 mA step current input and subsequent termination of signal (solid lines). The empirical population firing rates originating from Forbes et al. [28] (located in Fig 3A) are overlayed with corresponding colors. The lower portion of the transfer function was fit to match the step response in the 0-40s range. Note that the step response afferent firing rate data stems from a different population of afferents than those used to generate the empirical data in panel b, likely contributing to some overshoot of the transfer functions during the simulated step response. c, The gain and phase responses of the extended transfer functions, which more closely match the phase data from Kwan et al. [29] near 0.1Hz for both regular and irregular afferents. For comparisons to the phase data, which was not segregated for anodal and cathodal stimulation types, averages of the anodal and cathodal transfer function phases were taken. Cathodal and Anodal phase responses are shown in the background with consistent colors to the gain response curves. d, The four irregular/regular cathodal/anodal transfer functions, which now span a greater frequency range thus enabling modeling of both DC and dynamic (up to 25 Hz) GVS evoked firing rates.