Department of Neurobiology and Anatomy and the Center for Visual Science, University of Rochester, Rochester, New York 14642, USA
The primate linear VOR (LVOR) includes two forms. First, eye-movement
responses to translation [e.g., horizontal responses to interaural
(IA) motion] help maintain binocular fixation on targets, and
therefore a stable bifoveal image. The translational LVOR is
strongly modulated by fixation distance, and operates with high-pass
dynamics (>1 Hz). Second, other LVOR responses occur that
cannot be compensatory for translation and instead seem compensatory
for head tilt. This reflects an otolith response ambiguity-that
is, an inability to distinguish head translation from head tilt
relative to gravity. Thus, ocular torsion is appropriately compensatory
for head roll-tilt, but also occurs during IA translation, since
both stimuli entail IA acceleration. Unlike the IA-horizontal
response, IA torsion behaves with low-pass dynamics (with respect
to "tilt"), and is uninfluenced by fixation distance. Interestingly,
roll-tilt, like IA translation, also produces both horizontal
(a translational reflex) and torsional (a tilt reflex) responses,
further emphasizing the ambiguity problem. Early data from subjects
following unilateral labyrinthectomy, which demonstrates a general
immediate decline in translational LVOR responses, are also
presented, followed by only modest recovery over several months.
Interestingly, the usual high-pass dynamics of these reflexes
shift to an even higher cutoff. Both eyes respond roughly equally,
suggesting that unilateral otolith input generates a binocularly
symmetric LVOR.
