RESPONSES OF PIGEON VESTIBULOCEREBELLAR NEURONS TO OPTOKINETIC STIMULATION .1. FUNCTIONAL-ORGANIZATION OF NEURONS DISCRIMINATING BETWEEN TRANSLATIONAL AND ROTATIONAL VISUAL FLOW

1. Extracellular recordings were made from 235 neurons in the vestibul ocerebellum (VbC), including the flocculus (lateral VbC), nodulus (fol ium X), and ventral uvula (ventral folium lXc,d), of the anesthetized pigeon, in response to an optokinetic stimulus. 2. The optokinetic sti muli consisted of two black and white random-dot patterns that were ba ck-projected onto two large tangent screens. The screens were oriented parallel to each other and placed on either side of the bird's head. The resultant stimulus covered the central 100-degrees x 100-degrees o f each hemifield. The directional tuning characteristics of each unit were assessed by moving the largefield stimulus in 12 different direct ions, 30-degrees apart. The directional tuning curves were performed m onocularly or binocularly. The binocular directional tuning curves wer e performed with the direction of motion the same in both eyes (in-pha se; e.g., ipsi = upward, contra - upward) or with the direction of mot ion opposite in either eye (antiphase; e.g., ipsi = upward, contra = d ownward). 3. Mossy fiber units (n = 17) found throughout folia IXa,b a nd IXc,d had monocular receptive fields and exhibited direction select ivity in response to stimulation of either the ipsilateral (n = 12) or contralateral (n = 5) eye. None had binocular receptive fields. 4. Th e complex spike (CS) activity of 218 Purkinje cells in folia lXc,d and X exhibited direction selectivity in response to the large-field visu al stimulus moving in one or both visual fields. Ninety-one percent of the cells had binocular receptive fields that could be classified int o four groups: descent neurons (n = 112) preferred upward motion in bo th eyes; ascent neurons (n = 14) preferred downward motion in both eye s: roll neurons (n = 33) preferred upward and downward motion in the i psilateral and contralateral eyes, respectively; and yaw neurons (n = 40) preferred forward and backward motion in the ipsilateral and contr alateral eyes, respectively. Within all groups, most neurons (70%) sho wed an ipsilateral dominance. 5. For most binocular neurons (91%), the maximum depth of modulation occurred with simultaneous stimulation of both eyes, compared with monocular stimulation of the dominant eye al one. For the translation neurons (descent and ascent), binocular in-ph ase stimulation produced the maximum depth of modulation, whereas for the rotation neurons (roll and yaw), binocular anti-phase stimulation produced the maximum depth of modulation. 6. There was a clear functio nal segregation of the translation and rotation neurons. Descent and a scent neurons were found in the medial VbC (ventral uvula and nodulus) , whereas roll and yaw neurons were found in the lateral VbC (flocculu s).