A. Schematic experimental design. The cyclopean axis (c) is defined by the middle of both left and right eye rotation axis, a1 and a2 respectively. Both a1 and a2 also designates the interocular distance (a). θ corresponds to the subject's head position relative to the screen axis. Along the horizontal axis crossing the screen, (e1) corresponds to the centre of the screen, namely the fixation point, and (e2) represents a given point on the screen. Both α and β correspond to the retinal angles of the segment [e1e2] seen by the left and the right eye, respectively. The distance (δ) between the middle of the cyclopean axis (c) and the fixation point (ε1), in other words the distance between the screen and the subject was 1 m in the present study. B. Plot of the magnitude of disparity for each point of the checkerboard and for each eye's deviation [-20°, -10°, 10°, 20°] normalized by the disparity calculated for the central eye position (θ = 0°, see methods for details of the calculation).