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1. Single-unit responses to large-field visual stimuli were recorded extracellularly from neurons in the dorsal cap of Kooy of the inferior olive in anesthetized, paralyzed rabbits. The visually modulated neurons in the dorsal cap responded optimally to slow rotation of random dot stimuli, which were produced using handheld patterns or a planetarium projector. 2. Neurons had either monocular or binocular receptive fields. For binocular receptive field neurons, monocular stimulation of one eye, called the dominant eye, elicited greater modulation than did stimulation of the other eye. Rotation about a particular axis, called the preferred axis, produced both maximal excitation and inhibition. On the basis of differences in preferred axis orientation and in eye dominance, three neuron classes called Vertical Axis, Anterior (45 degrees) Axis, and Posterior (135 degrees) Axis were distinguished. 3. Vertical Axis neurons were modulated exclusively from the eye contralateral to the inferior olive recording site. This cell type responded strongly to large-field visual stimuli rotating about the vertical axis. Excitation resulted from movement in the temporal to nasal direction, and inhibition occurred during movement in the nasal to temporal direction. 4. Two subclasses of Anterior (45 degrees) Axis neurons were distinguished according to whether the receptive field was monocular or binocular. For both subclasses, the dominant eye was ipsilateral. The receptive field organization of the dominant eye was bipartite as described in the previous paper (51) for neurons in the midbrain visual tegmental relay zone. Anterior (45 degrees) Axis neurons were maximally excited when the stimulus pattern moved upward and posterior above the horizon in the anterior quadrant of the ipsilateral visual field, from 0 degrees (nose) to approximately 45 degrees azimuth. From 45 to 180 degrees azimuth (occiput) and above the horizon, these neurons were excited by downward and posterior movement. Inhibition occurred with oppositely directed movements. For rotating stimuli presented to the dominant eye, this class of neurons responded best to rotation of the visual world about an axis oriented near the horizontal plane and approximately 45 degrees azimuth. 5. The receptive field of Posterior (135 degrees) Axis neurons was always binocular, with the dominant eye contralateral. For the contralateral receptive field, from 0 degree (nose) to 135 degrees azimuth and above the horizon, excitation occurred during upward and posterior movement.(ABSTRACT TRUNCATED AT 400 WORDS)

1. To compare the spatial organization of the direction selectivity of neurons in the medial terminal nucleus (MTN) of the accessory optic system with that of neurons in the adjacent ventral tegmentum, extracellular single-unit recordings were made in the anesthetized rabbit. The ventral tegmental neurons were located in a region called the visual tegmental relay zone (VTRZ), which is defined by the ventral tegmental terminal field of contralaterally projecting MTN neurons. 2. Some of the present sample of MTN neurons (5 of 34) had monocular receptive fields composed of two parts distinguished by a marked difference in the orientation of their respective direction-selective tuning curves. For one part of the receptive field the preferred excitatory direction was "up," while for the other part it was "down." Such receptive fields for one eye were called bipartite, whereas the more usually encountered MTN receptive fields, which could be characterized by a single direction-selective tuning curve, were called uniform. 3. Of the 16 neurons recorded from the VTRZ, all but one were binocular. For these neurons, both uniform and bipartite receptive fields were found for each eye alone. The only monocular neuron encountered in the VTRZ had a contralateral, bipartite receptive field. 4. The spatial organization of the direction selectivity of bipartite receptive fields strongly suggests that they are suited to represent rotation of the visual field about a horizontal axis located in the vertical plane that divides the receptive field into two parts. 5. The boundary between the two parts of the bipartite receptive fields was found using handheld visual stimuli at one of two azimuthal locations, either close to 45 degrees or between 95 and 125 degrees (the 0 degree reference was rostral in the midsagittal plane). This particular structure of the bipartite receptive fields suggests that their preferred rotation axes have a close spatial relation to the best-response axes of the semicircular canals. 6. Seven VTRZ neurons were antidromically activated by electrical stimulation of the ipsilateral dorsal cap of the inferior olive. Since the receptive fields of VTRZ neurons have many of the structural features characteristic of the receptive fields of rostral dorsal cap neurons we conclude that the spatial organization of the receptive fields of dorsal cap neurons is, for the most part, synthesized prior to the inferior olive.(ABSTRACT TRUNCATED AT 400 WORDS)