Non-human primates exhibit disconjugate ocular counterroll to head roll tilts

To investigate the effect of head roll tilt on the binocular coordination of ocular counterroll in non-human primates, we measured binocular ocular counterroll in two rhesus monkeys fixating a straight ahead target, while adopting different head roll tilt positions. We used two infrared cameras to take snapshots of the left and the right eye in order to measure the resulting ocular counterroll responses. The horizontal and vertical components of the position of one of the two eyes where measured using an implanted 2D-search coil in one monkey and video-based eye tracking in the second one. We consistently observed disconjugate ocular counterroll responses to static head roll in both monkeys. Invariably, the eye positioned further away from ground level by roll tilting the head always exhibited larger ocular counterroll than the other eye. The pattern of disconjugacy of the ocular counterroll responses exhibited by rhesus monkey parallels the one described for humans. The correspondence between the two species suggests that monkeys may serve as useful models in studies of the neuronal underpinnings of tilt-induced ocular counterroll and the perceptual compensation of uncompensated retinal image tilt.