OCULAR RESPONSES TO TRANSLATION AND THEIR DEPENDENCE ON VIEWING DISTANCE .1. MOTION OF THE OBSERVER

1. The horizontal eye movements induced by acceleration along the interaural axis were recorded from five monkeys (Macaca mulatta) by the use of the electromagnetic search-coil technique. Animals sat on a sled that was moved briefly in darkness along a linear track (bell-shaped acceleration profile: peak, 400 mm/s2; duration, 200 ms). Immediately before acceleration, animals fixated one of five target lights located at distances ranging from 16 to 150 cm. During fixation, the horizontal positions of both eyes were used to check vergence, while accommodation was monitored with an infrared optometer. 2. Sled motion induced eye movements that were generally smooth and compensatory, e.g., rightward sled motion elicited leftward eye movement. We attribute these responses to a translational vestibuloocular reflex (TVOR) that senses the sled motion through the otolith organs. However, in three animals, these responses were preceded by weak anticompensatory movements (duration, < 40 ms; amplitude, < 10% of the maximum compensatory response). 3. Geometry indicates that, during brief sled motion, the eye movements required to keep gaze aligned on a particular location ("full" compensation) are inversely proportional to the viewing distance. Response measures based on the computed eye velocity profiles, such as the velocity achieved at specific times ("time slices") or the peak values of the estimated covariance functions, all indicated that compensatory responses were a linear function of the inverse of the prior viewing distance. Cross-correlation analyses indicated that the effect of the prior viewing distance was to scale responses, although detailed spectral analyses revealed that high-frequency components (> 10 Hz) tended to scale less vigorously than lower ones. 4. The adequacy of the compensatory eye movements was assessed by calculating the gain (response recorded/response required for full compensation). Regardless of the response measure used, gains varied considerably from one animal to another and, in some particular animals, from one direction to another but showed a general tendency to increase with viewing distance. For example, on the basis of the peak eye velocity achieved within 250 ms of the onset of sled motion, mean gain at 16 cm was 0.74 (range, 0.48-1.01), whereas at 150 cm it was 1.25 (range, 0.67-1.73). 5. Using wedge prisms to dissociate vergence and accommodation indicated that ocular responses to sled motion were sensitive to selective increases in either vergence (base-out prism with the most distant target) or accommodation (base-in prism with the nearest target). However, the magnitude of the effects showed considerable variability from one animal to another and, in some particular animals, from one direction to another. Further, neither vergence nor accommodation alone nor a linear combination of the two cues could account for all of the data. 6. We conclude that 1) translational accelerations can elicit robust compensatory eye movements that show the required linear dependence on proximity, although with considerable intersubject variability in the impoverished conditions used here (darkness); 2) vergence and accommodation cues are used to modulate compensatory eye movements in accordance with viewing distance, but they are often not the only cues used.