Relative Orientation of Primary Positions of the Two Eyes

We evaluated the relative orientations of the displacement planes of the two eyes under various conditions: fixations of nearby targets, of far targets and targets presented dichoptically at optical infinity. We show that disparity driven vergence is not always required to rotate the primary positions. We find that eye orientation during fixation of far targets is idiosyncratic. We found a bimodal distribution ranging from null to about 30 deg of the relative exorotations of the two primary positions. By contrast, the difference of primary positions' orientation of the two eyes was, for targets at optical infinity, stable and similar across subjects. However, the displacement planes of the two eyes did not coincide, but were exorotated by 4.3 deg on average, even though horizontal vergence was close to zero. We discuss our results with reference to current models of binocular three-dimensional control.