Effects of Canal Plugging on the Vestibulo-ocular Reflex and 2 Vestibular Nerve Discharge during Passive and Active Head

30 Mechanical occlusion (plugging) of the slender ducts of semicircular canals has been 31 used in the clinic as well as in basic vestibular research. Here, we investigated the effect 32 of canal plugging in two macaque monkeys on the horizontal vestibulo-ocular reflex 33 (VOR) and the responses of vestibular-nerve afferents during passive head rotations. 34 Afferent responses to active head movements were also studied. The horizontal VOR 35 gain decreased after plugging to < 0.1 for frequencies < 2 Hz but rose to ~0.6 as 36 frequency was increased to 15 Hz. Afferents innervating plugged horizontal canals had 37 response sensitivities which increased with the frequency of passive rotations from < 0.01 38 (spikes/s)/(/s) at 0.5 Hz to values of about 0.2 and 0.5 (spikes/s)/(/s) at 8 Hz for regular 39 and irregular afferents, respectively (<50% of responses in controls). An increase in 40 phase lead was also noted following plugging in afferent discharge, but not in the VOR. 41 Because the phase discrepancy between the VOR and afferent discharge is much larger 42 than seen in control animals, this suggests that central adaptation shapes VOR dynamics 43 following plugging. The effect of canal plugging on afferent responses can be modeled 44 as an increase in stiffness and a reduction in the dominant time constant and gain in the 45 transfer function describing canal dynamics. Responses were also evident during active 46 head rotations, consistent with the frequency content of these movements. We conclude 47 that canal plugging in macaques is only effective at frequencies <2 Hz. At higher 48 frequencies, afferents show significant responses with nearly a 90 degrees phase lead 49 such that they encode near rotational acceleration. Our results demonstrate that afferents 50 innervating plugged canals respond robustly during voluntary movements, a finding that 51 has implications for understanding the effects of canal plugging in clinical practice. 52 53 54 55 56