Characteristics of extreme-gaze ocular fixation in sparse visual surroundings

The purpose of this dissertation was to determine the characteristics of extreme-gaze ocular fixation. This was accomplished by examining eye position and movements during fixation and scleral search coil slippage. Subject calibration of search coil equipment was also studied. Seven young, emmetropic subjects fixated an LED target monocularly in the dark. A bite bar was used to fix the head.;Using search coil equipment, eye positions were collected as subjects fixated a 6 arcmin LED through nasal eye rotation angles (0°, 10°, 20°, 30° and 40°). A foveal afterimage was used to monitor for stress-induced eccentric fixation, however none was detected. Even after correction for search coil slippage, temporal fixation lags increased with eye rotation angle, and the mean eye position measured at 40° significantly lagged the target (14.5 arcmin). Fixation precision did not appear to change over eye rotation angles. Overall, while measurements with the scleral search coil suggested a lag of fixation, target foveation occurred at all gaze angles. This imperfect representation of the fovea's position by the search coil equipment may have been due to retinal stretching or crystalline lens tilting. Typical microsaccades occurred in all subjects, and end-gaze nystagmus occurred in two subjects during 40° eye rotations.;High-magnification photography was used to directly measure slippage of the scleral search coil during nasal gaze angles. Search coil slippage was always in the temporal direction, progressively increased with eye rotation angle, and was significant during eye rotations of 30° (11.9 arcmin) and 40° (23.3 arcmin). The data suggest that the scleral lens and bulbar conjunctiva slide together during eye movements, and return to their original position as the eye returns to straight-ahead gaze.;At extreme angles, search coil calibrations from a mechanical device differed from calibrations collected from human subjects. For search coil-based experiments where the eye is rotated laterally >10°, it may be more accurate to calibrate with subjects rather than the mechanical device. This would eliminate confounding influences from search coil slippage, improper target positioning and imperfect representation of the fovea's position by the search coil equipment.