SPATIOTEMPORAL RESPONSE PROPERTIES OF CEREBELLAR PURKINJE-CELLS TO NECK DISPLACEMENT

The activity of 184 Purkinje cells and 58 unidentified neurons located within the cerebellar anterior vermis was recorded in decerebrate cat s during wobble of the body under a fixed head. This stimulus induced a neck displacement of constant amplitude (2.5 degrees) whose directio n rotated at the constant velocity of 56.2 degrees/s on the horizontal plane, both in the clockwise and counterclockwise directions. It was then possible to evaluate the spatiotemporal characteristics of unit r esponses to neck displacement in the vertical planes; 131 of 184 Purki nje cells (71%) and 35 of 58 unidentified cells (60%) responded to clo ckwise and/or counterclockwise rotations. In particular, among the res ponsive units, 44% of the Purkinje cells and 37% of the unidentified c ells showed an equal amplitude modulation during clockwise and counter clockwise rotations. These units are expected ro show a maximal respon se sensitivity for neck displacement in a preferred direction, a null response for perpendicularly oriented stimuli and a constant temporal phase (narrowly tuned neurons). In 28% of the Purkinje cells and 40% o f the unidentified cells, responses of different amplitudes were obser ved during clockwise and counterclockwise rotations. These neurons sho uld display a preferred direction oi response to neck displacement, la ck of null response directions and a temporal phase changing with the stimulus direction (broadly tuned neurons). Finally, 27% of the Purkin je cells and 23% of the unidentified cells responded only to wobble in the clockwise or counterclockwise direction (unidirectional units). T his behavior predicts dual sensitivities for all the directions of nec k displacement and a response phase changing linearly with the directi on of neck displacement. A maximal sensitivity vector (S-max), aligned with the preferred direction of the neuron, was evaluated for the bid irectional narrowly tuned and broadly tuned units. Its amplitude and t emporal phase corresponded to the response characteristics expected fo r stimuli in the preferred direction of the cell. S-max directions wer e distributed over the horizontal plane. Most of them, however, were c loser to the pitch than to the roll axis and pointed towards the anima l's tail. Among pitch-related Purkinje cells, the temporal phase of S- max was small with a predominance of phase lags; phase leads of rather large amplitude were usually observed for roll-related Purkinje cells . The possibility that the recorded population of units coded the dire ction of neck displacement was tested by assuming that each cell gave a vectorial contribution related to its response properties and that t he vectorial sum of such contributions represented the outcome of the population code. Dynamic body-to-head displacements in four different directions were simulated and for each direction 12 population vectors were evaluated at regular intervals of the stimulus cycle. The direct ion of the population vector was related to that of the stimulus, but the correspondence was close only fbr the pitch direction. Moreover, t he amplitude of the population vector depended upon the direction of t he stimulus, being larger for pitch than for roll displacements. Due t o the efferent connections of the explored cerebellar region, the neur onal signals generated by the Purkinje cell population are probably tr ansferred to the spinal cord, where they may differentially affect the amplitude and the spatial properties of the neck reflexes according t o the direction of neck displacement. (C) 1998 LBRO. Published by Else vier Science Ltd.