Asymmetric adaptive gain changes of the vertical vestibulo-ocular reflex in cats

The present study was conducted to examine adaptive gain changes of vertical vestibulo-ocular reflex (VOR) after exposure to a vertical visual-vestibular mismatch in cats. The visual-vestibular mismatch was induced by oscillating the animals for an hour about an inter-aural axis at frequencies of 0.16 and 0.32 Hz with the peak velocity of 20 degrees/s, coupled with either in-phase ("gain decrease" conditioning) or out-of-phase ("gain increase" conditioning) sinusoidal rotation of a random-dot pattern. Eye movements were measured with a magnetic search coil system. Before conditioning, vertical VOR showed up-down asymmetric responses. That is, upward slow phase eye velocity (SPV) in response to downward head rotation was significantly larger than downward SPV in response to upward head rotation. After adaptation to "gain increase" conditioning, VOR gain increased in both stimulus directions. The increase in VOR gain was significantly larger for upward SPV than for downward SPV. After adaptation to "gain decrease" conditioning, VOR gain decreased in both stimulus directions. The decrease in VOR gain was, however, significantly larger for downward SPV than for upward SPV. Our results indicate that VOR in the vertical plane adaptively changes but that the gain change shows a directional asymmetry. This asymmetry was dependent on the direction of the slip of visual image rather than the direction of head rotation, and the gain change was smaller when the retinal slip was generated downward. Possible explanations for the asymmetry are discussed on a physiological and anatomical basis.