Gaze shifts evoked by electrical stimulation of the superior colliculus inthe head-unrestrained cat. II. Effect of muscimol inactivation of the caudal fastigial nucleus

The medioposterior cerebellum [vermian lobules VI and VII and caudal fastig ial nucleus (cFN)] is known to play a major role in the control of saccadic gaze shifts toward a visual target. To determine the relative contribution of the cFN efferent pathways to the brainstem reticular formation and to t he superior colliculus (SC), we recorded in the head-unrestrained cat the e ffects of cFN unilateral inactivation on gaze shifts evoked by electrical m icrostimulation of the deeper SC layers. Gaze shifts evoked after muscimol injection still exhibited the typical qualitative features of normal saccad ic gaze shifts. Nevertheless, consistent modifications in amplitude and lat ency were observed. For ipsiversive movements (evoked by the SC contralater al to the inactivated cFN), these changes depended on the locus of stimulat ion on the motor map: for the anterior 2/3 of the SC, amplitude increased a nd latency tended to decrease; for the posterior 1/3 of the SC, amplitude d ecreased and latency increased. For the contraversive direction, amplitude moderately decreased and latency tended to increase for all but the caudal- most stimulated SC site. These modifications of SC-evoked gaze shifts durin g cFN inactivation differed from the ipsiversive hypermetria/contraversive hypometria pattern observed for visually triggered gaze shifts recorded dur ing the same recording sessions. We conclude that (i) the topographical org anization of gaze shift amplitude in the deeper SC layers is influenced by the cerebellum and is either severely distorted or demonstrates an amplitud e reduction during inactivation of the contralateral or ipsilateral cFN, re spectively; (ii) gaze shifts evoked by SC microstimulation and visually tri ggered gaze shifts either rely on distinct cerebellar-dependent control pro cesses or differ by the location of the caudal-most active SC population. W e present a functional scheme providing several predictions regarding the m odulatory influence of the cerebellum on SC neuronal activities and on the topographical organization of fastigial-SC projections.