Adaptive plasticity in the naso-occipital linear vestibulo-ocular reflex.

The linear vestibulo-ocular reflex (LVOR) during motion along the naso-occipital (NO) axis is governed by eye position and viewing distance. These influences are necessary for the LVOR to maintain stable foveal images during head translation. The response to NO translation must be large when eye position is eccentric from the axis of head motion (i.e., during lateral gaze) and must diminish as eye position approaches straight-ahead, eventually reaching zero when the eye is aligned with the NO axis of motion (the "null point"). As eye position crosses to the opposite side, the LVOR response must reappear, but in the opposite direction, and must grow in magnitude as eccentricity increases. To determine whether the NO-LVOR is subject to adaptive plastic mechanisms, squirrel monkeys were conditioned during NO translation while they binocularly viewed a rich visual field through parallel base-right or base-left wedge prisms. This optical method effectively shifted the visual world 9 degrees leftward or rightward, respectively, thus inducing a mismatch between vision and the NO-LVOR during head movements. To restore compensatory function, the relationship between LVOR sensitivity and horizontal eye position must shift by 9 degrees in the same direction as the visual image shift, effectively shifting the null point. After 2 h of adaptive conditioning, all monkeys exhibited an adaptive shift in the appropriate direction by an average of 3.0 degrees (range 0.7-5.0 degrees), corresponding to 33% of the geometrically required adaptation.