To stabilize objects of interest on the fovea during translation, vestibular-driven compensatory eye movements [translational vestibulo-ocular reflex (TVOR)] must scale with both target distance and eccentricity. To identify the neural correlates of these properties, we recorded from different groups of eye movement-sensitive neurons in the prepositus hypoglossi and vestibular nuclei of macaque monkeys during lateral and fore-aft displacements. All neuron types exhibited some increase in modulation amplitude as a function of target distance during high-frequency (4 Hz) lateral motion in darkness, with slopes that were correlated with the cell's pursuit gain, but not eye position sensitivity. Vergence angle dependence was largest for burst-tonic (BT) and contralateral eye-head (EH) neurons and smallest for ipsilateral EH and position-vestibular-pause (PVP) cells. On the other hand, the EH and PVP neurons with ipsilateral eye movement preferences exhibited the largest vergence-independent responses, which would be inappropriate to drive the TVOR. In addition to target distance, the TVOR also scales with target eccentricity, as evidenced during fore-aft motion, where eye velocity amplitude exhibits a "V-shaped" dependence and phase shifts 180° for right versus left eye positions. Both the modulation amplitude and phase of BT and contralateral EH cells scaled with eye position, similar to the evoked eye movements during fore-aft motion. In contrast, the response modulation of ipsilateral EH and PVP cells during fore-aft motion was characterized by neither the V-shaped scaling nor the phase reversal. These results show that distinct premotor cell types carry neural signals that are appropriately scaled by vergence angle and eye position to generate the geometrically appropriate compensatory eye movements in the translational vestibulo-ocular reflex.
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