DORSAL Y-GROUP IN THE SQUIRREL-MONKEY .1. NEURONAL RESPONSES DURING RAPID AND LONG-TERM MODIFICATIONS OF THE VERTICAL VOR

1. The activity of 113 Y group neurons was recorded extracellularly in 5 alert squirrel monkeys. Sixty-two cells were recorded in naive anim als, and 51 cells were recorded after adaptation of the vestibuloocula r reflex (VOR) with the use of telescopic lenses. The animals were lyi ng on their right side, so that head rotation was in the vertical (pit ch) plane and optokinetic mulation elicited vertical eye movement. The responses of most cells, as well as the concurrent eye movement, were studied during 1) the VOR, elicited in darkness or in light by sinuso idal head rotation, 2) visual following, elicited by sinusoidal rotati on of a full-field optokinetic drum around the stationary animal, and 3) paradigms of visual-vestibular interaction, elicited by combined si nusoidal vestibular and optokinetic simulation. Stimulation parameters for both head and drum velocity were usually 0.5 Hz, 35 degrees/s pea k velocity. 2. Y group cells respond vigorously during visual followin g and during suppression of the VOR (produced by in-phase rotation of the head and the optokinetic drum): the response is approximately in-p hase with eye velocity during visual following, and approximately in-p hase with head velocity during suppression of the VOR. During the VOR in darkness. Y cells usually exhibit only slight modulation. The resul ts suggest a linear interaction of visual following and vestibular sig nals on Y cells during vertical visual-vestibular interaction. Taking into account the excitatory projection of Y cells to superior rectus a nd inferior oblique motoneurons, a causal role of the Y group in rapid modification of VOR ain during visual-vestibular interaction is sugge sted. 3. Nine Y neurons from two animals were recorded continuously, f or periods ranging from 30 min to 5 h, while the VOR was being adapted to higher or lower gain. Progressive changes of the gain of the VOR i n darkness were evident after similar to 30 min from ;he initiation of head rotation under visual-vestibular mismatch. Consistent changes of the gain and/or phase of the neuronal response during the VOR in dark ness were noted in all cases. The phase of the neuronal response gradu ally approximated head velocity phase during adaptation of the VOR to low gain, increases in the neuronal gain thereafter ensued; the opposi te changes were observed during adaptation of the VOR to high gain. 4. Sixteen Y cells were recorded from 1 animal chronically adapted to hi gh VOR gain with the use of magnifying lenses, and 35 cells were recor ded from 2 animals chronically adapted to low VOR gain with the use of miniaturizing lenses. After chronic adaptation of the VOR, the vast m ajority of Y cells responded vigorously during the VOR in darkness; th e response was approximately out-of-phase with head velocity after ada ptation of the VOR to high gain, and approximately in-phase with head velocity after adaptation of the VOR to low gain. 5. Because the Y gro up monosynaptically excites extraocular motoneurons mediating upward e ye movement, the observed changes in the responses of Y cells during t he VOR in darkness are in the ''correct'' direction to cause at least part of the behavioral adaptation. A role of the Y group in adaptive r egulation of the gain of the vertical VOR is suggested.