Responses of Purkinje cells in the cerebellar anterior vermis to off-vertical axis rotation.

Responses of 67 Purkinje cells (P-cells) and 44 unidentified neurons (U-cells) located in the cerebellar anterior vermis were recorded in decerebrate cats during off-vertical axis rotation (OVAR). This stimulus consisted of a slow constant velocity (9.4%/s) rotation in the clockwise (CW) and counterclockwise (CCW) directions around an axis inclined by 5 degrees with respect to the vertical. OVAR imposes on the animal head a 5 degrees tilt, whose direction changes continuously over the horizontal plane, thus eliciting a selective stimulation of macular receptors. A total of 27/67 P-cells (40%) and 24/44 U-cells (55%) responded to both CW and CCW rotations. For these bidirectional units, the direction of maximum sensitivity to tilt (Smax) could be identified. Smax directions were distributed over the whole horizontal plane of stimulation. Among bidirectional neurons, 48% of the P-cells and 33% of the U-cells displayed an equal amplitude of modulation during CW and CCW rotations, indicating a cosine-tuned behaviour. In these instances, the temporal phase of the unit response to a given direction of tilt remained constant, while the sensitivity was maximal along the Smax direction and declined with the cosine of the angle between Smax and the tilt direction. The remaining bidirectional units displayed unequal amplitudes of modulation during CW and CCW rotations. For these neurons, a nonzero sensitivity along the null direction was expected and the response phase varied as a function of stimulus direction. Finally, 31% and 23% of P-cells and U-cells, respectively, responded during OVAR in one direction only (unidirectional units). This behaviour predicts equal sensitivities along any tilt direction in the horizontal plane and a response phase that changes linearly with the stimulus direction. The posibility that the tested neurons formed a population which coded the direction of head tilt in space was also investigated. The data from the whole population of cells were analysed using a modified version of vectorial analysis. This model assumes that for a particular tilt each cell makes vectorial contributions; the vectorial sum of these contributions represent the outcome of the population code and points in the direction of head tilt in space. Thus, a dynamic head tilt along four representative directions was simulated. For each of the four directions, 12 population vectors were calculated at regular time intervals so as to cover an entire cycle of head tilt. The results indicate that for each selected time in the cycle the direction of the population vector closely corresponded to that of the head tilt, while its amplitude was related to the amount of head tilt. These data were particularly obtained for the P-cells. In view of their efferent connections, the cerebellar anterior vermis may provide a framework for the spatial organization of vestibulospinal reflexes induced by stimulation of otolith receptors.