SPATIOTEMPORAL RESPONSE PROPERTIES OF CEREBELLAR PURKINJE-CELLS TO ANIMAL DISPLACEMENT - A POPULATION ANALYSIS

The hypothesis that corticocerebellar units projecting to vestibulospi nal neurons contribute to the spatiotemporal response characteristics of forelimb extensors to animal displacement was tested in decerebrate cats in which the activity of Purkinje cells and unidentified cells l ocated in the cerebellar anterior vermis was recorded during wobble of the whole animal. This stimulus imposed to the animal a tilt of fixed amplitude (5 degrees)dagger with a direction moving at a constant ang ular velocity (56.2 degrees/s), both in the clockwise and counterclock wise directions over the horizontal plane. Eighty-three percent (143/1 73) of Purkinje cells and 81% (42/52) of unidentified cells responded to clockwise and/or counterclockwise rotations. In particular, 116/143 Purkinje cells (81%) and 32/42 unidentified cells (76%) responded to both clockwise and counterclockwise relations (bidirectional units), w hile 27/143 Purkinje cells (19%) and 10/42 unidentified cells (24%) re sponded to wobble in one direction only (unidirectional units). For th e bidirectional units, the direction of maximum sensitivity to tilt (S -max) was identified. Among these units, 24% of the Purkinje cells and 26% of the unidentified cells displayed an equal amplitude of modulat ion during clockwise and counterclockwise rotations, indicating a cosi ne-tuned behavior. For this unit type, the temporal phase of the respo nse to a given direction of tilt should remain constant, while the sen sitivity would be maximal along the S-max direction, declining with th e cosine of the angle between S-max and the tilt direction. The remain ing bidirectional units, i.e. 57% of the Purkinje cells and 50% of the unidentified cells displayed unequal amplitudes of modulation during clockwise and counterclockwise rotations. For these neurons, a non-zer o sensitivity along the null direction is expected, with a response ph ase varying as a function of stimulus direction. As to the unidirectio nal units, their responses to wobble in one direction predict equal se nsitivities along any tilt direction in the horizontal plane and a res ponse phase that changes linearly with the stimulus direction. By comp aring these data with those obtained previously during selective stimu lation of macular receptors by a 5 degrees off-vertical axis rotation, it appeared that the directions of maximum sensitivity to tilt were d istributed over the whole horizontal plane of stimulation, in both con ditions. However, co-stimulation of macular and canal receptors during wobble decreased the proportion of unidirectional units, while that o f the bidirectional, namely broadly tuned units, increased. In additio n, while the average gain of the S-max vector of the bidirectional uni ts was comparable, the temporal phase of this vector tended to show a more prominent phase leading behavior during wobble with respect to of f-vertical axis rotation. The possibility that the tested neurons form ed a population which coded the direction of head till in space was al so investigated using a modified version of the classical population v ector analysis. It was shown that for each selected time in the tilt c ycle the direction of the population vector closely corresponded to th at of the head tilt, while its amplitude was related to that of the st imulus. We conclude that the broad distribution of the response vector orientation of units located in the cerebellar anterior vermis repres ents an appropriate substrate for the cerebellar control of vestibulos pinal reflexes involving extensor muscles during a variety of head til ts. In view of their efferent projections to the vestibular and fastig ial nuclei, the cerebellar anterior vermis may provide a framework for the spatial coding of vestibular inputs, giving equal emphasis to bot h side-to-side and fore-aft stability. (C) 1997 IBRO. Published by Els evier Science Ltd.