Semicircular canal plugging shifts the frequency of spatial-temporal convergence (STC) in central vestibular neurons.

INTRODUCTION Oscillating the head about a spatial horizontal axis activates both the vertical semicircular canals (VC) and the otolith organs. It is generally assumed that in-phase modulation of unit firing rate (FR) with head position indicates convergent otolith input, while in-phase modulation with head velocity indicates VC input. The amplitude of FR modulation depends on head orientation in yaw and decreases to zero at some head orientations, but the phase remains relatively constant, although it shifts 180° after passing through the zero point [1, 2]. Some neurons have significant modulation of FR in all head orientations in yaw, while the phase of the response varies from being in-phase with stimulus velocity in one orientation and in-phase with head position in positions orthogonal to the first head orientation. Such unit response behaviour is called a spatio-temporal convergence (STC) characteristic [3], and it is postulated to arise from convergent VC and otolith inputs [3, 4] or from two otolith inputs with different dynamic properties [5]. It was recently demonstrated that dynamic otolith could mimic VC-induce responses [6]. This implies that STC characteristics that were previously thought to be due to canal-otolith interaction could be due to otolith-otolith interactions. Plugging does not completely inactivate semicircular canals but instead decreases the cupula/endlymph time constant from 4 s to ≈0.07 s [7]. This implies that plugged canals have substantial response to oscillation at 1-4 Hz and above. Canal plugging, however, should not compromise otolith function. Thus, if STC behaviour of neurons is due to otolith-otolith interaction, than it should be present at low frequencies even after canal plugging. On the contrary, if STC characteristics arise from canalotolith interaction, than neurons should have STC characteristics only at frequencies that activate the semicircular canals after plugging. How canal plugging affects STC characteristics of neurons was tested in the present study. The experiments conformed to the Guide for the Care and Use of Laboratory Animals and were approved by the IACUC.