Context compensation in the vestibuloocular reflex during active head rotations

The vestibuloocular reflex (VOR) needs to modulate its gain depending on ta rget distance to prevent retinal slip during head movements. We investigate d gain modulation (context compensation) for binocular gaze stabilization i n human subjects during voluntary yaw and pitch head rotations. Movements o f each eye were recorded, both when attempting to maintain gaze on a small visual target at straight-ahead in a darkened room and after its disappeara nce (remembered target). In the analysis, we relied on a binocular coordina te system yielding a version and a vergence component. We examined how freq uency and target distance, approached here by using vergence angle, affecte d the gain and phase of the version component of the VOR and compared the r esults to the requirements for ideal performance. Linear regression analysi s on the version gain-vergence relationship yielded a slope representing th e influence of target proximity and an intercept corresponding to the respo nse at zero vergence ("default gain"). The slope of the fitted relationship , divided by the geometrically required slope, provided a measure for the q uality of version context compensation ("context gain"). In both yaw and pi tch experiments, we found default version gains close to one even for the r emembered target condition, indicating that the active VOR for far targets is already close to ideal without visual support. In near target experiment s, the presence of visual feedback yielded near unity context gains, indica ting close to optimal performance (retinal slip <0.4<degrees>/s). For remem bered targets, the context gain deteriorated but was still superior to perf ormance in corresponding passive studies reported in the literature. In gen eral, context compensation in the remembered target paradigm was better for vertical than for horizontal head rotations. The phase delay of version ey e velocity relative to head velocity was small (similar to2 degrees) for bo th horizontal and vertical head movements. Analysis of the vergence data fr om the near target experiments showed that context compensation took into a ccount that the two eyes require slightly different VORs. In the DISCUSSION , comparison of the present default VOR gains and context gains with data f rom earlier passive studies has led us to propose a limited role for effere nce copies during self-generated movements. We also discuss how our analysi s can provide a framework for evaluating two different hypotheses for the g eneration of binocular VOR eye movements.