The objective of these experiments is to understand how the brain incorporates viewing distance into a coordinated motor response of the eyes and head during vertical accelerations. Over the past 10 years it has been demonstrated that ocular responses to linear motion are inversely proportional to viewing distance. This fact holds true regardless of whether the motion is due to translational motion of the visual scene, mediated by the ocular following reflex or due to translational head motion, mediated by the translational vestibulo-ocular reflex (TVOR). Both reflexes result in compensatory ocular responses which preserve the stability of the image on the retina. However, these studies were all conducted with the animal's head restrained, a condition that is not representative of normal experience. When the head is free to move, changes in the visual axis are associated with movements of both the eyes and the head (i.e., gaze). Studies using humans and non-human primates have demonstrated marked differences in the eye movements evoked during rotational motion when head-free and head-restrained results were compared. The proposed studies will examine the compensatory eye and head movements in """"""""head-free"""""""" primates exposed to brief vertical translations. The primates will be trained to fixate on a small target light positioned at various distances. Specific questions will address 1) how the effect of viewing distance is parsed between the command signals driving the eyes and the head during a compensatory gaze shift in the dark (translational gaze reflex, TGR), 2) whether anticompensatory inertial movements of the eyes, due to transient accelerations are dampened when the head is free to move, and 3) the latency of the vertical TGR. The Vertical Free-Fall Sled (VFS) produces a transient vertical linear acceleration characterized by an abrupt onset followed by a gradual deceleration. My previous work using the VFS, made use of this property to provide the first accurate measurement of the latency of the vertical TVOR in rhesus monkeys. In the proposed study, rapid onset of the stimulus will allow us to examine the compensatory visual and vestibular responses prior to the onset of visual feedback. Because an abrupt onset to the stimulus is achievable, it will be possible to characterize the temporal and spatial parsing of this signal - a necessary step towards understanding gaze control during normal behaviors such as locomotion where viewing distance is not static.
