Directional asymmetry in vertical smooth-pursuit and cancellation of the vertical vestibulo-ocular reflex in juvenile monkeys |
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Authors: | Teppei Akao Yousuke Kumakura Sergei Kurkin Junko Fukushima Kikuro Fukushima |
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Institution: | (1) Department of Physiology, Hokkaido University School of Medicine, West 7, North 15, Sapporo 060-8638, Japan;(2) Present address: Department of Health Sciences, Hokkaido University School of Medicine, Sapporo, Japan |
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Abstract: | Young primates exhibit asymmetric eye movements during vertical smooth-pursuit across a textured background such that upward
pursuit has low velocity and requires many catch-up saccades. The asymmetric eye movements cannot be explained by the un-suppressed
optokinetic reflex resulting from background visual motion across the retina during pursuit, suggesting that the asymmetry
reflects most probably, a low gain in upward eye commands (Kasahara et al. in Exp Brain Res 171:306–321, 2006). In this study, we examined (1) whether there are intrinsic differences in the upward and downward pursuit capabilities
and (2) how the difficulty in upward pursuit is correlated with the ability of vertical VOR cancellation. Three juvenile macaques
that had initially been trained only for horizontal (but not vertical) pursuit were trained for sinusoidal pursuit in the
absence of a textured background. In 2 of the 3 macaques, there was a clear asymmetry between upward and downward pursuit
gains and in the time course of initial gain increase. In the third macaque, downward pursuit gain was also low. It did not
show consistent asymmetry during the initial 2 weeks of training. However, it also exhibited a significant asymmetry after
4 months of training, similar to the other two monkeys. After 6 months of training, these two monkeys (but not the third)
still exhibited asymmetry. As target frequency increased in these two monkeys, mean upward eye velocity saturated at ∼15°/s,
whereas horizontal and downward eye velocity increased up to ∼40°/s. During cancellation of the VOR induced by upward whole
body rotation, downward eye velocity of the residual VOR increased as the stimulus frequency increased. Gain of the residual
VOR during upward rotation was significantly higher than that during horizontal and downward rotation. The time course of
residual VOR induced by vertical whole body step-rotation during VOR cancellation was predicted by addition of eye velocity
during pursuit and VOR x1. These results support our view that the directional asymmetry reflects the difference in the organization
of the cerebellar floccular region for upward and downward directions and the preeminent role of pursuit in VOR cancellation. |
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Keywords: | Smooth-pursuit Vestibular system VOR cancellation Gain Directional asymmetry Training Juvenile macaque Cerebellar flocculus |
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