Representations of ocular rotations in the cerebellar flocculus of the rabbit

The climbing fibres (CFs) of the rabbit flocculus that respond in a speed- and direction-selective manner to retinal image slip produced by eye rotations can be divided into three classes on the basis of the orientation of the rotation axis associated with their greatest modulation (the preferred axis). The similarity of the orientations of these axes to those of the eye rotation axes of the extraocular muscles suggests that a simple geometrical correspondence may exist between the eye rotation associated with the preferred axis of a given class of CFs and the eye rotation produced by activation of the Purkinje cells upon which that class of CFs synapse. To pursue this possibility, the axes of the eye rotations evoked by electrical microstimulation of the alert rabbit's flocculus were determined simultaneously for both eyes in three dimensions using two orthogonal search coils on each eye. A limited number of slow eye movement response patterns were found, and of these, two predominated. The most common response was a counterclockwise (CCW) rotation of the ipsilateral (left) eye around an axis close to the horizontal plane and at about 140 degrees posterior to the nose. The other predominant response was abduction of the ipsilateral eye. These two response patterns, together with the smaller conjugate components for the contralateral eye, are consonant with the orientations of the preferred CF axes. In addition, a clear CCW rotation of the contralateral (right) eye about its 135 degrees axis was also evoked from some stimulation sites. This response, which occurred either alone or as a component of an upward rotation about the nasal-occipital (roll) axis, is at variance with the orientations of the preferred CF axis. However, the latencies of the CCW contralateral 135 degrees component (80-140 ms) were greater than those of the CW contralateral 45 degrees component, the CCW ipsilateral 135 degrees component and the ipsilateral abduction component (8-48 ms). These latency differences may distinguish stimulation of Purkinje cells from stimulation of other neurones.