A review of mechanical and synaptic processes in otolith transduction of sound and vibration for clinical VEMP testing

Older studies of mammalian otolith physiology have focused mainly on sustained responses to low-frequency (<50Hz) or maintained linear acceleration. So the otoliths have been regarded as accelerometers. Thus evidence of otolithic activation and high precision phase locking to high-frequency sound and vibration appears to be very unusual. However those results are exactly in accord with a substantial body of knowledge of otolith function in fish and frogs. It is likely that phase locking of otolith afferents to vibration is a general property of all vertebrates. This review examines the literature about the activation and phase locking of single otolithic neurons to air-conducted sound (ACS) and bone-conducted vibration (BCV), in particular the high precision of phase locking shown by mammalian irregular afferents which synapse on striolar type I hair cells by calyx endings. Potassium in the synaptic cleft between the type I hair cell receptor and the calyx afferent ending may be responsible for the tight phase locking of these afferents even at very high discharge rates. Since frogs and fish do not possess full calyx endings, it is unlikely that they show phase locking with such high precision and to such high frequencies as has been found in mammals. The high-frequency responses have been modelled as the otoliths operating in a seismometer mode rather than an accelerometer mode. These high-frequency otolithic responses constitute the neural basis for clinical vestibular-evoked myogenic potential (VEMP) tests of otolith function.