Monovision vs Modified Monovision: Binocular Cortical Suppression, Stereoacuity & Prism Disparities

Monovision induces intentional anisometropia: the dominant eye is corrected for distance ($0.00\ \text{D}$ vergence demand), while the non-dominant eye receives a plus add ($+1.50\ \text{D}$ to $+2.25\ \text{D}$).

The visual cortex relies on interocular blur suppression. When fixating at distance, clear foveal input from the dominant eye is processed while blurred input from the non-dominant eye is cortically suppressed via striate lateral inhibition. At near, the reverse occurs.

Clinical success hinges on accurate dominance determination:

• Sighting (Motor) Dominance: Evaluated via the Hole-in-Card (Miles/Dolman) test. The eye aligning with the aperture when alternate eyes are occluded is sighting dominant.
• Sensory Dominance: Evaluated by placing a $+1.50\ \text{D}$ fogging lens alternately over each eye during binocular distance viewing. The eye experiencing greater subjective blur is sensory dominant.

Golden Rule: Distance correction must be assigned to the sensory dominant eye. Forcing distance correction onto a sighting-dominant but sensory-weak eye triggers visual rivalry and severe asthenopia.

Monovision reduces normal stereoacuity ($15\text{--}25$ arcsec) down to $100\text{--}400$ arcsec, eliminating the $41\%$ binocular contrast summation gain.

Modified Monovision remedies this: the dominant eye receives distance correction, while the non-dominant eye receives a center-near simultaneous multifocal lens. This preserves intermediate computer screen clarity ($60\text{--}80\ \text{cm}$), reduces interocular anisometropia to $< 1.00\ \text{D}$, and retains stereopsis at $\le 60$ arcseconds.