Dynamics of Squirrel Monkey Linear Vestibuloocular Reflex and Interactions
with Fixation Distance
Laura Telford, Scott H. Seidman, and Gary D. Paige
Department of Neurology and Center for Visual Science
University of Rochester, Rochester, New York
APStracts 4:091N, 1997.
ABSTRACT
Horizontal, vertical and torsional eye movements were recorded using the
magnetic search-coil technique during linear accelerations along the
interaural (IA) and dorsoventral (DV) head axes. Four squirrel monkeys were
translated sinusoidally over a range of frequencies (0.5-4.0 Hz) and
amplitudes (0.1-0.7 g peak acceleration). The linear vestibulo-ocular reflex
(LVOR) was recorded in darkness following brief presentations of visual
targets at various distances from the subject. With subjects positioned
upright or nose-up relative to gravity, IA translations generated conjugate
horizontal (IA-horizontal) eye movements, while DV translations with the head
nose-up or right-side down generated conjugate vertical (DV-vertical)
responses. Both were compensatory for linear head motion and are thus
translational LVOR responses. In concert with geometric requirements, both IA-
horizontal and DV-vertical response sensitivities (in deg. eye rotation/cm
head translation) were linearly related to reciprocal fixation distance as
measured by vergence (in m-1, or meter-angles, MA). The relationship was
characterized by linear regressions, yielding sensitivity slopes (in
deg/cm/MA) and intercepts (sensitivity at zero vergence). Sensitivity slopes
were greatest at 4.0 Hz, but were only slightly more than half the ideal
required to maintain fixation. Slopes declined with decreasing frequency,
becoming negligible at 0.5 Hz. Small responses were observed when vergence was
zero (intercept), although no response is required. Like sensitivity slope,
the intercept was largest at 4.0 Hz and declined with decreasing frequency.
Phase lead was near zero (compensatory) at 4.0 Hz, but increased as frequency
declined. Changes in head orientation, motion axis (IA vs. DV) and
acceleration amplitude produced slight and sporadic changes in LVOR
parameters. Translational LVOR response characteristics are consistent with
high-pass filtering within LVOR pathways.
Along with horizontal eye movements, IA translation generated small torsional
responses. In contrast to the translational LVORs, IA-torsional responses were not systematically modulated by vergence angle. The IA-torsional LVOR is not
compensatory for translation since it cannot maintain image stability. Rather, it likely compensates for the effective head tilt simulated by translation.
When analyzed in terms of effective head tilt, torsional responses were
greatest at the lowest frequency and declined as frequency increased,
consistent with low-pass filtering of otolith input. It is unlikely that IA-
torsional responses compensate for actual head tilt, however, since they were
similar for both upright and nose-up head orientations. The IA-torsional and
IA-horizontal LVORs seem to respond only to linear acceleration along the IA
head axis, and the DV-vertical LVOR to acceleration along the head's DV axis,
regardless of gravity.
Received 24 January 1997; accepted in final form 13 June 1997.
APS Manuscript Number J0069-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997