Elsevier

Neuroscience

Volume 118, Issue 2, 8 May 2003, Pages 563-570
Neuroscience

Research paper
Effects of reversible shutdown of the monkey flocculus on the retention of adaptation of the horizontal vestibulo-ocular reflex

https://doi.org/10.1016/S0306-4522(02)00991-0Get rights and content

Abstract

There are two different proposals regarding the role of the cerebellar flocculus in the adaptation of the vestibulo-ocular reflex: that the flocculus is the site for both the induction and retention of the adaptation, or that the flocculus plays an important role in the induction, but the vestibular nuclei to which the flocculus issues its efferences are the site of retention. To locate the memory trace for the adaptation of the vestibulo-ocular reflex, we determined effects of acute pharmacological shutdown of the bilateral cerebellar flocculi in four Macaca fuscata. The gain of the horizontal vestibulo-ocular reflex was measured by sinusoidal oscillation of the turntable by 10° (peak-to-peak) at 0.33 Hz in the dark. Two hours of 0.33 Hz–10° sinusoidal oscillation of the turntable while viewing the stationary checked-pattern screen through a ×2.2 magnifying lens adaptively increased the horizontal vestibulo-ocular reflex gain by 0.2 on the average. The gain increase lasted for 1 h when the monkeys were left with their heads fixed in darkness, but it disappeared within 24 h after the monkeys were returned to their cages where they had free movements. The effects of injections of 5 or 10% lidocaine chloride into the bilateral floccular areas (8 μl for each side) immediately after adaptation occurred were determined in nine sessions. The gain of the horizontal vestibulo-ocular reflex immediately decreased to the level before the adaptation. These effects of lidocaine lasted for at least 1 h. On the contrary, injections of the same amount of Ringer’s solution, tested in eight sessions, hardly affected gain of the horizontal vestibulo-ocular reflex over 1-h period. These depressant effects of lidocaine injections were specific to the gains increased by adaptation. These results suggest that the memory trace for the short-term adaptation of the horizontal vestibulo-ocular reflex gain induced by 2 h of sustained visual-vestibular interactions resides within the flocculus.

Section snippets

Abbreviations used in the figures

    DP

    dorsal paraflocculus

    FL

    flocculus

    FR

    Fresnel lens

    Lat

    lateral

    LP

    lobulus petrosus of the paraflocculus

    PLF

    postero-lateral fissure which demarcates flocculus and ventral paraflocculus

    TT

    turntable

    VN

    vestibular nuclei

    VO

    vestibular organ

    VP

    ventral paraflocculus

    VIII

    nerve VIII

Experimental procedures

Three males (monkeys TO, IR and MA) and one female (monkey YU) Macaca fuscata (body weight 5–7 kg) were used. Under 40 mg/kg of sodium pentobarbital (Nembutal, Dainippon, Tokyo, Japan) plus 5 mg/kg of ketamine (Ketalar, Sankyo, Tokyo, Japan) anesthesia and sterile conditions, the monkeys were surgically fitted with four anchor bolts and a cylinder for head fixation, and two chambers for drug injection. Sclera-search coil was also implanted in the left eye. After a 1-week recovery period, the

Adaptation of the HVOR

Collectively, adaptation of the HVOR was investigated in 17 sessions for the four monkeys; six for monkey TO, six for monkey IR, four for monkey YU and one for monkey MA. For monkey TO, IR and YU, a lidocaine solution was injected in half of the sessions, and the same amount of Ringer’s solution was injected in the remaining half. For monkey MA, only a lidocaine solution was injected. The gains and phases of the HVOR before the adaptation were 0.91±0.01 and 6.9±1.2° (n=6) for monkey TO,

Discussion

The present study reveals that 2 h of sustained visual–vestibular interactions induced adaptive gain increase in monkeys, which lasted at least for 1 h when the monkey’s head was fixed in darkness. This adaptive effect in the HVOR gain disappeared immediately after the local injections of 8 μl of 5 or 10% lidocaine chloride into the flocculi. These effects of lidocaine on the HVOR gain continued for 1 h. On the contrary, injections of Ringer’s solution into the flocculi hardly affected the HVOR

Acknowledgements

The authors are very grateful to Dr. Masao Ito (RIKEN, BSI) for his encouragement and valuable suggestions throughout the course of experiment and preparation of the manuscript.

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