(1) To assess the adequacy of the magnetic field coil method (MFCM) for measures of extreme gaze ocular fixation; and (2) to measure the accuracy of ocular fixation for a short duration at gaze angles of 10 to 40°.
Seven subjects participated. Two experiments were performed on the same day. In both experiments, the head and eye were rotated in opposite directions. In the first experiment, search coil slippage was assessed using photographs of a scleral search coil worn on one eye. These photographs were taken with a slitlamp biomicroscope equipped with a digital photography system as subjects rotated the eyes through nasal angles up to 40°. In the second experiment, measures of ocular position were made using the MFCM as the eye was rotated through nasal angles up to 40° to fixate a light-emitting diode target under two conditions. In one condition, subjects simply aimed the eye at the target. In the other, subjects placed a foveal afterimage on the target as the target was viewed through a pinhole.
Temporal search coil slippage >10 arc min was noted for three subjects. For one subject, this slippage occurred at gaze angles ≥20°, whereas for the other subjects, the slippage appeared at gaze angles of ≥30°. Ocular fixation had to be assessed from the differences in fixation position between the two conditions in the second experiment because of search coil slippage and because the relationship between the anterior and posterior portions of the eyes changed for some individuals as the gaze angle changed. Fixation was usually accurate (within 10 arc min). There were occasional instances where fixation errors >15 arc min occurred, but we believe that all but one of these cases were attributable to poor localization of the afterimage.
Scleral search coils can slip on some individuals in extreme gaze. Search coils may also be inadequate to indicate the direction of the fovea in extreme gaze for some individuals because the relationship between the anterior and posterior portions can change with gaze angle. Once artifacts associated with the MFCM and localization of the afterimage are accounted for, ocular fixation is almost always within 10 arc min of the fixation target at gaze angles of 10 to 40°.