Department of Surgery (Otolaryngology) and Anatomy, University of Mississippi Medical Center, 2500 North State Street, Jackson, Mississippi 39216-4505, USA
Translational vestibulo-ocular reflexes (trVORs) are characterized
by distinct spatio-temporal properties and sensitivities that
are proportional to the inverse of viewing distance. Anodal
(inhibitory) labyrinthine stimulation (100 µA, >2 s)
during motion decreased the high-pass filtered dynamics, as
well as horizontal trVOR sensitivity and its dependence on viewing
distance. Cathodal (excitatory) currents had opposite effects.
Translational VORs were also affected after unilateral labyrinthectomy.
Animals lost their ability to modulate trVOR sensitivity as
a function of viewing distance acutely after the lesion. These
deficits partially recovered over time, albeit a significant
reduction in trVOR sensitivity as a function of viewing distance
remained in compensated animals. During fore-aft motion, the
effects of unilateral labyrinthectomy were more dramatic. Both
acute and compensated animals permanently lost their ability
to modulate fore-aft trVOR responses as a function of target
eccentricity. These results suggest that (1) the dynamics and
viewing distance-dependent properties of the trVORs are very
sensitive to changes in the resting firing rate of vestibular
afferents and, consequently, vestibular nuclei neurons; (2)
the most irregularly firing primary otolith afferents that are
most sensitive to labyrinthine electrical stimulation might
contribute to reflex dynamics and sensitivity; (3) inputs from
both labyrinths are necessary for the generation of the translational
VORs.
