OCULAR RESPONSES TO TRANSLATION AND THEIR DEPENDENCE ON VIEWING DISTANCE .2. MOTION OF THE SCENE

1. The ocular following responses induced by brief (100-ms) movements of the visual scene were examined for their dependence on viewing distance in 5 monkeys (Macaca mulatta). The horizontal positions of both eyes and the vertical position of one eye were recorded using the electromagnetic search-coil technique. Accommodation was monitored in selected experiments by use of an infrared optometer. Test patterns (random dots) were back-projected onto a translucent tangent screen facing the animal. Six viewing distances were used (range, 20-150 cm), the size and speed of the image on the screen being adjusted for each so as to preserve a constant retinal image. 2. Response measures based on the amplitude of the first peak in the eye acceleration profile or the eye velocity achieved at specific times all indicated that ocular following responses were inversely related to viewing distance, the relationship being linear for the earliest measures. On average, the sensitivity to viewing distance was comparable with that reported for the translational vestibuloocular reflex (TVOR) in the preceding paper: as viewing distance increased from 20 cm, ocular following decremented at a mean rate (+/- SD) of 17 +/- 3% per m-1, while the TVOR decremented at a mean rate (+/- SD) of 18 +/- 1% per m-1. 3. Ocular following responses showed the postsaccadic enhancement described by Kawano and Miles regardless of viewing distance. To a first approximation, the effects of postsaccadic enhancement and viewing distance summed linearly. 4. The dependence of ocular following on speed showed the progressive saturation previously described by Miles et al. at all viewing distances, the peak eye velocity achieved being inversely related to the viewing distance, indicating that the saturation must originate upstream of the dependence on viewing distance. Under normal viewing conditions, this speed saturation will tend to offset the dependence on viewing distance because the retinal slip speeds experienced by the moving observer will tend to vary inversely with viewing distance, resulting in greater saturation with nearer viewing. 5. Wedge prisms were used to dissociate vergence and accommodation and indicated that ocular following responses were sensitive to selective increases in either vergence (base-out prism with the screen at 100 cm) or accommodation (base-in prism with the screen at 20 cm). However, as with the TVOR, 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. Our data are consistent with the idea that the early component of ocular following and the TVOR share some common structure (a central pathway with efficacy modulated by various cues to viewing distance) and function (compensation for translational disturbances of the observer).