DYNAMICS OF SQUIRREL-MONKEY LINEAR VESTIBULOOCULAR REFLEX AND INTERACTIONS WITH FIXATION DISTANCE

Horizontal, vertical, and torsional eye movements were recorded using the magnetic search-coil technique during linear accelerations along t he interaural (IA) and dorsoventral (DV) head axes. Four squirrel monk eys 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 after brief presentation s of visual targets at various distances from the subject. With subjec ts positioned upright or nose-up relative to gravity, IA translations generated conjugate horizontal (IA horizontal) eye movements, whereas DV translations with the head nose-up or right-side down generated con jugate vertical (DV vertical) responses. Both were compensatory for li near head motion and are thus translational LVOR responses. In concert with geometric requirements, both IA-horizontal and DV-vertical respo nse sensitivities (in deg eye rotation/cm head translation) were relat ed linearly to reciprocal fixation distance as measured by vergence (i n m(-1), or meter-angles, MA). The relationship was characterized by l inear regressions, yielding sensitivity slopes (in deg . cm(-1). MA(-1 )) and intercepts (sensitivity at 0 vergence). Sensitivity slopes were greatest at 4.0 Hz, but were only slightly more than half the ideal r equired to maintain fixation. Slopes declined with decreasing frequenc y, becoming negligible at 0.5 Hz. Small responses were observed when v ergence was zero (intercept), although no response is required. Like s ensitivity slope, the intercept was largest at 4.0 Hz and declined wit h 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 a nd sporadic changes in LVOR parameters. Translational LVOR response ch aracteristics are consistent with high-pass filtering within LVOR path ways. Along with horizontal eye movements, IA translation generated sm all torsional responses. In contrast to the translational LVORs, IA-to rsional responses were not systematically modulated by vergence angle. The IA-torsional LVOR is not compensatory for translation because 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 lowes t frequency and declined as frequency increased, consistent with low-p ass filtering of otolith input. It is unlikely that IA-torsional respo nses compensate for actual head tilt, however, because they were simil ar for both upright and nose-up head orientations. The IA-torsional an d -horizontal LVORs seem to respond only to linear acceleration along the LA head axis, and the DV-vertical LVOR to acceleration along the h ead's DV axis, regardless of gravity.