Conditions of measuring fixation disparity.

(A) Without prisms, the objective fixation disparity (oFD) is the difference between the observed vergence angle V during binocular recording and the stimulus vergence angle V₀, which is geometrically given by the viewing distance D and the interpupillary distance p, i.e., V₀ = 2 arctan (p/2D). In this example of an over-convergent (eso) oFD, V is larger than V₀. The monocular components of V₀ are measured during the eye tracker calibration that is made separately for the left and right eye: the eye position during monocular fixation represents the zero position for the subsequent binocular recording period. The covered eye assumes the heterophoria resting state. For an optical correction of an eso fixation disparity (as in this example), base-out prisms are applied. These prisms turn the visual axes optically outward (drawn lines), which requires the eye muscles to converge more (broken lines) to maintain fusion. (B) When prisms are applied, V_0P = Prismpower + V₀ is the stimulus vergence angle and V_p is the vergence angle. The subjective fixation disparity is illustrated by the angular amount of horizontal offset d_Non between two dichoptically presented nonius lines (sFD = arctan (d_Non/D), d_Non > 0); this offset has to be adjusted on the test monitor so that the observer perceives the two nonius lines in alignment. Subjective fixation disparity is typically smaller than the objective fixation disparity, as indicated in the graph. The graphs show the case of visual axes that intersect in front of the fixation point when the fixation cross is projected on the nasal part of the retina within Panum’s area; this over-convergent state is referred to as eso fixation disparity with a positive sign. In the opposite under-convergent state, the visual axes intersect behind the fixation point, the fixation cross is projected on the temporal part of the retina within Panum’s area and the nonius lines have a reversed position (d_Non < 0, exo fixation disparity with a negative sign); in the latter case, base-in prisms are applied. (C) Schematic diagram of a typical “fixation disparity curve”, i. e., fixation disparity is plotted as a function of forced vergence when prisms are placed in front of the eyes. Conventionally, positive signs are used for base-out prisms and negative signs for base-in prisms.