MODULATION OF VERGENCE BY OFF-VERTICAL YAW AXIS ROTATION IN THE MONKEY - NORMAL CHARACTERISTICS AND EFFECTS OF SPACE-FLIGHT

Horizontal movements of both eyes were recorded simultaneously using s cleral search coils in 2 rhesus monkeys before and after the COSMOS 22 29 spaceflight of 1992-1993. Another 9 monkeys were tested at comparab le time intervals and served as controls. Ocular vergence, defined as the difference in horizontal position between the left and right eyes, was measured during off-vertical yaw axis rotation (OVAR) in darkness . Vergence was modulated sinusoidally as a function of head position w ith regard to gravity during OVAR. The amplitude of peak-to-peak modul ation increased with increments in tilt of the angle of the rotational axis (OVAR tilt angle) that ranged from 15 degrees to 90 degrees. Of the II monkeys tested, I had no measurable modulation in vergence. In the other 10, the mean amplitude of the peak to peak modulation was 5. 5 degrees+/-1.3 degrees at 90 degrees tilt. Each of these monkeys had maximal vergence when its nose was pointed close to upward (gravity ba ck; mean phase: -0.9 degrees+/-26 degrees). After space flight, the mo dulation in vergence was reduced by over 50% for the two flight monkey s, but the phase of vergence modulation was not altered. The reduction in vergence modulation was sustained for the 11-day postflight testin g period. We conclude that changes in vergence are induced in monkeys by the sinusoidal component of gravity acting along the naso-occipital axis during yaw axis OVAR, and that the modulation of the vergence re flex is significantly less sensitive to linear acceleration after spac e night.