HUMAN GAZE STABILIZATION DURING NATURAL ACTIVITIES - TRANSLATION, ROTATION, MAGNIFICATION, AND TARGET DISTANCE EFFECTS

Stability of images on the retina was determined in 14 normal humans i n response to rotational and translational perturbations during self-g enerated pitch and yaw, standing, walking, and running on a treadmill. The effects on image stability of target distance, vision, and specta cle magnification were examined. During locomotion the horizontal and vertical velocity of images on the retina was <4 degrees/s for a visib le target located beyond 4 m. Image velocity significantly increased t o >4 degrees/s during self-generated motion. For all conditions of sta nding and locomotion, angular vestibule-ocular reflex (AVOR) gain was less than unity and varied significantly by activity, by target distan ce, and among subjects. There was no significant correlation (P > 0.05 ) between AVOR gain and image stability during standing and walking de spite significant variation among subjects. This lack of correlation i s likely due to translation of the orbit. The degree of orbital transl ation and rotation varied significantly with activity and viewing cond ition in a manner suggesting an active role in gaze stabilization. Orb ital translation was consistently antiphase with rotation at predomina nt frequencies <4 Hz. When orbital translation was neglected in comput ing gaze, computed image velocities increased. The compensatory effect of orbital translation allows gaze stabilization despite subunity AVO R gain during natural activities. Orbital translation decreased during close target viewing, whereas orbital rotation decreased while wearin g telescopic spectacles. As the earth fixed target was moved closer, i mage velocity on the retina significantly increased (P < 0.05) for all activities except standing. Latency of the AVOR increased slightly wi th decreasing target distance but remained <10 ms for even the closest target. This latency was similar in darkness or light, indicating tha t the visual pursuit tracking is probably not important in gaze stabil ization. Trials with a distant target were repeated while subjects wor e telescopic spectacles that magnified vision by 1.9 or 4 times. Gain of the AVOR was enhanced by magnified vision during all activities, bu t always to a value less than spectacle magnification. Gain enhancemen t was greatest during self-generated sinusoidal motion at 0.8 Hz and w as less during standing, walking, and running. Image slip velocity on the retina increased with increasing magnification. During natural act ivities, slip velocity with telescopes increased most during running a nd least during standing. Latency of the visually enhanced AVOR signif icantly increased with magnification (P < 0.05), probably reflecting a contribution of the visual pursuit system. The oculomotor estimate of target distance was inferred by measuring binocular convergence, as w ell as from monocular parallax during head translation. In darkness, t arget distance estimates obtained by both techniques were less accurat e than in light, consistently overestimating for near and underestimat ing for far targets.