Vertical ocular responses to constant linear acceleration generated by fore-aft head translation in monkeys.

We examined the vertical linear vestibuloocular reflexes (LVORs) elicited by constant linear acceleration (0-0.5 g for >95 ms) during transient fore-aft translation in three monkeys. In the dark condition, small but consistent downward ocular responses to forward translation were observed (latencies >41 ms) when the initial vertical eye positions were at 0 degrees , although eye movements following backward translation were inconsistent among animals. These downward ocular responses showed the following three characteristics: they were independent of vertical gaze eccentricities, their magnitudes were almost proportional to the forward acceleration, and they were reduced by the large-field (not the spot) visual information. These characteristics revealed that the downward ocular responses to forward translation were the tilt LVORs. In addition, we recognized that the translational LVOR, which depended on vertical gaze eccentricities, was working at the same time. Our data suggest that constant linear acceleration during forward translation evokes the tilt LVOR, as well as the illusory tilting perception.