Convergence of directional vestibular and neck signals on cerebellar Purkinje cells.

Abstract Convergence of spatially oriented vestibular and neck signals within the cerebellar anterior vermis in decerebrate cats was studied by recording the simple spike discharge of Purkinje (P) cells during wobble either of the whole animal (vestibular input) or of the body under a fixed head (neck input) at 0.156 Hz, 5 degrees and 2.5 degrees, respectively. Both clockwise (CW) and counterclockwise (CCW) rotations were performed. Units that had equal response amplitudes to CW and CCW rotations (narrowly tuned neurons) were described by a single vector (S-max), characterized by a gain, a direction and a temporal phase. Units with different response amplitudes to CW and CCW rotation (broadly tuned neurons) were described by two vectors (S-max and S-min). In addition to these bidirectional units, there were also unidirectional units which responded either to CW or CCW rotation; in these cases the gain of S-max equals that of S-min. On the whole, 77% and 63% of the P cells re spending to vestibular and neck stimulation, respectively, showed a bidirectional broadly tuned or unidirectional behavior. These response patterns were attributed to the convergence of signals with different spatial and temporal properties. About 50% of the P cells from which recordings were made responded to stimulation of both sensory systems. However, the gains of the S-max vectors of the neck responses were much greater than those of the vestibular responses, at least for small amplitudes of rotation, and were positively correlated with them. Usually the differences in the orientation components of the neck and vestibular S-max vectors were larger, while the differences in temporal phases were smaller than 90 degrees. These findings suggest that periodic changes in the phase difference and gain ratio of the neck to the vestibular response may occur during dynamic displacement of the head over the body, depending on the stimulus direction. As a result of these data, the P cells of the cerebellar vermis are expected to show prominent responses to head rotation, which could affect the spatially organized postural responses by utilizing vestibular and reticular targets.