Primate translational vestibuloocular reflexes. IV. Changes after unilateral labyrinthectomy

The effects of unilateral labyrinthectomy on the properties of the translat ional vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-l asting transient displacements. During small-amplitude, steady-state sinuso idal lateral oscillations, a small decrease in the horizontal trVOR sensiti vity and its dependence on viewing distance was observed during the first w eek after labyrinthectomy. These deficits gradually recovered over time. In addition, the vertical response component increased, causing a tilt of the eye velocity vector toward the lesioned side. During large, transient late ral displacements, the deficits were larger and longer lasting. Responses a fter labyrinthectomy were asymmetric, with eye velocity during movements to ward the side of the lesion being more compromised. The most profound effec t of the lesions was observed during fore-aft motion. Whereas responses wer e kinematically appropriate for fixation away from the side of the lesion ( e.g., to the left after right labyrinthectomy), horizontal responses were a nticompensatory during fixation at targets located ipsilateral to the side of the lesion (e.g., for targets to the right after right labyrinthectomy). This deficit showed little recovery during the 3-mo post-labyrinthectomy t esting period. These results suggest that inputs from both labyrinths are i mportant for the proper function of the trVORs, although the details of how bilateral signals are processed and integrated remain unknown.