Binocular 3D otolith-ocular reflexes: responses of normal chinchillas to tilt and translation.

Head rotation, translation, and tilt with respect to a gravitational field elicit reflexive eye movements that partially stabilize images of Earth-fixed objects on the retinas of humans and other vertebrates. Compared with the angular vestibulo-ocular reflex, responses to translation and tilt, collectively called the otolith-ocular reflex (OOR), are less completely characterized, typically smaller, generally disconjugate (different for the 2 eyes) and more complicated in their relationship to the natural stimuli that elicit them. We measured binocular 3-dimensional OOR responses of 6 alert normal chinchillas in darkness during whole body tilts around 16 Earth-horizontal axes and translations along 21 axes in horizontal, coronal, and sagittal planes. Ocular countertilt responses to 40-s whole body tilts about Earth-horizontal axes grew linearly with head tilt amplitude, but responses were disconjugate, with each eye's response greatest for whole body tilts about axes near the other eye's resting line of sight. OOR response magnitude during 1-Hz sinusoidal whole body translations along Earth-horizontal axes also grew with stimulus amplitude. Translational OOR responses were similarly disconjugate, with each eye's response greatest for whole body translations along its resting line of sight. Responses to Earth-horizontal translation were similar to those that would be expected for tilts that would cause a similar peak deviation of the gravitoinertial acceleration (GIA) vector with respect to the head, consistent with the "perceived tilt" model of the OOR. However, that model poorly fit responses to translations along non-Earth-horizontal axes and was insufficient to explain why responses are larger for the eye toward which the GIA vector deviates. As the first in a pair of papers on Binocular 3D Otolith-Ocular Reflexes, this paper characterizes binocular 3D eye movements in normal chinchillas during tilts and translations. The eye movement responses were used to create a data set to fully define the normal otolith-ocular reflexes in chinchillas. This data set provides the foundation to use otolith-ocular reflexes to back-project direction and magnitude of eye movement to predict tilt axis as discussed in the companion paper.