Effect of viewing distance on the generation of vertical eye movements during locomotion

Vertical head and eye coordination was studied as a function of viewing dis tance during locomotion. Vertical head translation and pitch movements were measured using a video motion analysis system (Optotrak 3020). Vertical ey e movements were recorded using a video-based pupil tracker (Iscan). Subjec ts (five) walked on a linear treadmill at a speed of 1.67 m/s (6 km/h) whil e viewing a target screen placed at distances ranging from 0.25 to 2.0 m at 0.25-m intervals. The predominant frequency of vertical head movement was 2 Hz. In accordance with previous studies, there was a small head pitch rot ation, which was compensatory for vertical head translation. The magnitude of the vertical head movements and the phase relationship between head tran slation and pitch were little affected by viewing distance, and tended to o rient the naso-occipital axis of the head at a point approximately 1 m in f ront of the subject (the head fixation distance or HFD). In contrast, eye v elocity was significantly affected by viewing distance. When viewing a far (2-m) target, vertical eye velocity was 180 degrees out of phase with head pitch velocity, with a gain of 0.8. This indicated that the angular vestibu le-ocular reflex (aVOR) was generating the eye movement response. The major finding was that, at a close viewing distance (0.25 m), eye velocity was i n phase with head pitch and compensatory for vertical head translation, sug gesting that activation of the linear vestibule-ocular reflex (IVOR) was co ntributing to the eye movement response. There was also a threefold increas e in the magnitude of eye velocity when viewing near targets, which was con sistent with the goal of maintaining gaze on target. The required vertical IVOR sensitivity to cancel an unmodified aVOR response and generate the obs erved eye velocity magnitude for near targets was almost 3 times that previ ously measured. Supplementary experiments were performed utilizing body-fix ed active head pitch rotations at 1 and 2 Hz while viewing a head-fixed tar get. Results indicated that the interaction of smooth pursuit and the aVOR during visual suppression could modify both the gain and phase characterist ics of the aVOR at frequencies encountered during locomotion. When walking, targets located closer than the HFD (1.0 m) would appear to move in the sa me direction as the head pitch, resulting in suppression of the aVOR. The r esults of the head-fixed target experiment suggest that phase modification of the aVOR during visual suppression could play a role in generating eye m ovements consistent with the goal of maintaining gaze on targets closer tha n the HFD, which would augment the IVOR response.