Ocular motor responses to abrupt interaural head translation in normal humans

We characterized the interaural translational vestibulo-ocular reflex (tVOR) in 6 normal humans to brief (∼200 ms), high-acceleration (0.4-1.4g) stimuli, while they fixed targets at 15 or 30 cm. The latency was 19 ± 5 ms at 15-cm and 20 ± 12 ms at 30-cm viewing. The gain was quantified using the ratio of actual to ideal behavior. The median position gain (at time of peak head velocity) was 0.38 and 0.37, and the median velocity gain, 0.52 and 0.62, at 15- and 30-cm viewing, respectively. These results suggest the tVOR scales proportionally at these viewing distances. Likewise, at both viewing distances, peak eye velocity scaled linearly with peak head velocity and gain was independent of peak head acceleration. A saccade commonly occurred in the compensatory direction, with a greater latency (165 vs. 145 ms) and lesser amplitude (1.8 vs. 3.2 deg) at 30- than 15-cm viewing. Even with saccades, the overall gain at the end of head movement was still considerably undercompensatory (medians 0.68 and 0.77 at 15- and 30-cm viewing). Monocular viewing was also assessed at 15-cm viewing. In 4 of 6 subjects, gains were the same as during binocular viewing and scaled closely with vergence angle. In sum the low tVOR gain and scaling of the response with viewing distance and head velocity extend previous results to higher acceleration stimuli. tVOR latency (∼20 ms) was lower than previously reported. Saccades are an integral part of the tVOR, and also scale with viewing distance.