Differential visual adaptation of vertical canal-dependent vestibulo-ocular reflexes.

Reversing vision in the horizontal (left-right) plane in humans induces adaptive mechanisms and even reversal of the horizontal vestibulo-ocular reflex (HVOR). The present experiments were aimed at investigating if such adaptive modifications could be observed in the frontal plane by reversal of the torsional visual world movements. Torsional vestibulo-ocular reflex (TVOR) was measured in one subject who wore Dove prisms for 19 days. The gain of TVOR was tested in the dark with the head leaned backward and rotating around an earth vertical axis with sinusoidal rotation (1/6 HZ). The gain decreased from 0.27 to 0.13 at 70 degrees peak-to-peak amplitude, and from 0.3 to 0.11 at 45 degrees peak-to-peak amplitude after 19 days of prism-wearing. Full gain recovery was observed 10 days after prism removal. The results are compared with the observation that in the same situation the vertical VOR (up-down) is not reversed (Dove prisms do not reverse visual images in this plane). As the same four (vertical) canals produce both reflexes, it is suggested that central neuronal mechanisms allow the recognition of the geometrical pattern of visual reversals and selectively adapt the reflex in the relevant planes.