Dynamic response to sound and vibration of the guinea pig utricular macula, measured in vivo using Laser Doppler Vibrometry

With the use of a commercially available Laser Doppler Vibrometer (LDV) we have measured the velocity of the surgically exposed utricular macula in the dorsoventral plane, in anaesthetized guinea pigs, during Air Conducted Sound (ACS) or Bone Conducted Vibration (BCV) stimulation. We have also performed simultaneous measurements of otolithic function in the form of the Utricular Microphonic (UM) and the Vestibular short-latency Evoked Potential (VsEP). Based on the level of macular vibration measured with the LDV, the UM was most sensitive to ACS and BCV between 100 and 200 Hz. The phase of the UM relative to the phase of the macular motion was relatively consistent across frequency for ACS stimulation, but varied by several cycles for BCV stimulation, suggesting a different macromechanical mode of utricular receptor activation. Moreover, unlike ACS, BCV evoked substantially distorted UM and macular vibration responses at certain frequencies, most likely due to complex resonances of the skull. Analogous to LDV studies of organ of Corti vibration, this method provides the means to study the dynamic response of the utricular macula whilst simultaneously measuring function.