Monovision vs Blended Vision: Interocular Suppression, Stereoacuity Degradation & 1.50D Anisometropia Limits

Monovision relies on interocular cortical suppression: when viewing a distant street sign, the visual cortex suppresses the blurred image from the near eye; when reading a menu, it suppresses the blurred distance eye:

$$\Delta F = F_{\text{near eye}} - F_{\text{dist eye}} \approx \mathbf{+2.00\text{ to } +2.50\ \text{D (Full Monovision)}}$$

However, when anisometropia exceeds $\mathbf{1.50\ \text{Diopters}}$, binocular fusion breaks down. Patients experience binocular rivalry (unpleasant visual alternating), spatial disorientation on staircases, and stereoacuity collapses from normal $40\text{ seconds of arc}$ down to $>200\text{ arcsec}$.

Rather than using crude monofocal defocus, Laser Blended Vision (Presbyond) combines micro-monovision with customized wavefront-induced spherical aberration:

1. Dominant Eye: Corrected for distance with slightly tailored negative spherical aberration ($Q \approx -0.30$), expanding individual depth of focus from distance down to $1.0\ \text{meter}$.
2. Non-Dominant Eye: Corrected with mild myopia (only $\mathbf{-1.25\text{ to } -1.50\ \text{D}}$) and controlled spherical aberration, stretching depth of focus from $33\ \text{cm}$ up to $1.0\ \text{meter}$.
3. The Seamless Intermediate Blend: Both eyes overlap perfectly at the intermediate zone ($60\text{ to } 100\ \text{cm}$), preserving $>90\%$ of natural stereoacuity with over $95\%$ patient adaptation rates.

Every candidate for surgical monovision or blended vision must undergo a mandatory 2-week soft contact lens simulation trial to evaluate personal cortical tolerance and verify binocular stereoscopic comfort before irreversible laser ablation.