The role of the cerebellum during motor learning in the vestibulo-ocular reflex: different mechanisms in different species?

Abstract

SIR: In a review article on p. 437 of this issue Lisberger argues that the discrepancy in the flocculus literature between rabbits and monkeys reflects species differences in basic principles of cerebellar function. As outlined in my recent review article 5, this assertion seems to be premature, since dif- ferent experimental approaches have been employed in different species. This pre- sents us with the challenge of employing approaches that have been useful in studies of the rabbit flocculus to the primate floc- culus, so that species comparisons can he made from comparable data bases. I would like to address, from this viewpoint, several specific points raised in Lisberger's article, Evolution of the flocculus As Lisberger mentions, eye movements in rabbits are mostly reflexive (vestibulo- ocular reflex and optokinetic response) and voluntary eye movements are very poorly developed, as contrasted to the well- developed saccades and smooth pursuit eye movements in primates. Lisberger suggests that this evolutionary change in eye move- ments was associated with a change in the functioning of the floccuhis. My suspicion, though, is that the basic circuitry for floccu- lar control of reflexive eye movements is retained in primates, even if it is buried among other floccular functions which evolved in connection with the develop- ment of voluntary eye movements. Anatomical and physiological studies have demonstrated that the flocculus consists of a number of microzones dedicated to differ- ent functions. In rabbits, a microzone which is presumed to play a specific role in the adaptive modification of the horizontal vestibulo-ocular reflex (VOR) has been located in a rostral area of the flocculus ~. Before accepting Lisberger's view, one has to confirm the absence of an equivalent microzone in the monkey flocculus. Microzones in monkeys One major problem is the lack of experi- mental data on the anatomical organization of the primate floccuhis. In particular, the absence of knowledge about the microzone structure of the floccuhis has been crucial. Balaban, Ito and Wantanahe ~ have used