Does the oculomotor system make use of high-level visual cues to viewing distance?

Depth judgments that require knowledge of viewing distance are strongly influenced by both vergence angle and by the pattern of vertical disparities across large visual fields. Given the established importance of disparity fields in depth perception, we hypothesized that the oculomotor system might also make use of high-level monocular and binocular cues to viewing distance. To address this hypothesis, we investigated how compensatory eye movements during translation (translational VOR) scale with viewing distance. Monkeys viewed random-dot stereograms with which we independently manipulated vergence angle, the vertical disparity field, relative horizontal disparities, and textural cues to viewing distance. Stereograms simulated a textured wall or pyramid at different distances, with stimuli rear-projected on a fixed screen 32cm in front of the animal (subtending 85 × 68 degrees). In a control condition, a dot field was projected onto a moveable screen at different distances from the animal. For all of these visual conditions, 5-10 cycles of left/right motion of the animal (at 5 Hz) were interleaved with 2-3 cycles in complete darkness. Only cycles for which fixation and vergence were well-controlled were admitted into analysis. For each stimulus condition, the relationship between tVOR gain and viewing distance was quantified by linear regression. As expected from previous work, tVOR gain depended strongly on vergence angle. Vertical disparity and texture cues had a statistically significant effect on tVOR gain, but these effects were ten-fold weaker than the effect of vergence angle. By contrast to the large effects that vertical disparities can have on depth judgments, our results suggest that the oculomotor system relies far less on high-level cues to viewing distance than does the perceptual system.