Binocular fixation in the rhesus monkey: spatial and temporal characteristics.

The horizontal and vertical components of the positions of both eyes of rhesus monkeys were measured during periods of binocularly stable eye positions (eye pauses) while the animals fixated a small target. Differences between monocular and binocular viewing, as well as effects of target size and background illumination, were assessed and found to be comparable to similar measures for humans. The scatter of eye position for either eye during binocular viewing had a standard deviation of 6-8 min arc in the horizontal and 7-13 min arc in the vertical meridia. Measurements of vergence and vertical misalignment, taken from binocular positional disparity, showed that for nearly 60% of eye pause time the eyes were misaligned on the fixation target by more than 7 min arc along both horizontal and vertical axes. In addition, the line of gaze during the trial was found to follow certain idiosyncratic tendencies for each monkey, although the positional variability remained relatively constant throughout the fixation trial. These observations suggest that during binocular fusion and stereopsis a mechanism exists that dynamically compensates for the relatively large shifts in retinal image position during fixation.