EXISTENCE IN THE NUCLEUS INCERTUS OF THE CAT OF HORIZONTAL-EYE-MOVEMENT-RELATED NEURONS PROJECTING TO THE CEREBELLAR FLOCCULUS

1. Properties of nucleus incertus (NIC) neurons projecting to the cere bellar flocculus were studied in alert cats by using chronic unit and eye movement recording and antidromic activation. Projection of these neurons onto the flocculus was verified with retrograde transport of h orseradish peroxidase after injections in the flocculus. 2. Bipolar st imulation electrodes were implanted into the ''middle'' zone of each f locculus because this zone is known to be involved in the control of h orizontal eye movements. The dorsomedial aspect of the pontine tegment um was explored with microelectrodes during stimulation of both floccu li. The majority of neurons antidromically activated from the flocculu s were found in the caudal part of the NIC. 3. Of the 69 neurons activ ated from the flocculus, 44 were classified as burst-tonic (BT) neuron s; 34 discharged in relation with horizontal movements of the eye, 10 in relation with vertical movements. Of the 14 remaining neurons, 6 we re not related to eye movements and 8 were classified as burst neurons . The BT neurons of the NIC displayed a great sensitivity to both hori zontal eye position and horizontal eye velocity. 4. This study demonst rates the presence of a new group of horizontal eye movement related B T neurons situated in the NIC. The fact that they project to the horiz ontal floccular zone emphasizes the importance of the functional speci alization of the different Purkinje cell zones.