The Preferred Retinal Locus (PRL): Microperimetry Fixation Stability & Eccentric Viewing

In geographic atrophy or disciform scarring from wet AMD, foveal cones undergo irreversible apoptosis, creating an Absolute Central Scotoma ($5^\circ\text{ to } 15^\circ$ diameter). When looking directly at an object, the target disappears into a blank dark void. Spontaneously over 3 to 6 months:

1. The visual cortex recruits an intact, functioning region of paramacular retina bordering the scotoma.
2. This area undergoes neural remapping, becoming the Preferred Retinal Locus (PRL)—the new center for fixation, oculomotor saccades, and pursuit movements.
3. In over $70\%$ of patients, the natural PRL establishes superior or superior-temporal to the fovea (representing the inferior visual field), which provides optimal left-to-right scanning during reading.

Standard visual field testing fails in low vision because patients cannot hold steady foveal fixation. Tracking Microperimetry (e.g., Nidek MP-3 or CenterVue MAIA) solves this via real-time infrared fundus tracking ($25\ \text{Hz}$):

$$\textbf{Fixation Stability: } \text{Bivariate Contour Ellipse Area (BCEA)} = 2 k \pi \sigma_H \sigma_V \sqrt{1 - \rho^2}$$

• The software plots thousands of retinal fixation points during testing, calculating the BCEA in square degrees ($\text{deg}^2$) representing the $68\%$ and $95\%$ confidence intervals.
• Stable Fixation: $> 75\%$ of fixation points fall within a $2^\circ$ circle ($BCEA < 1.5\ \text{deg}^2$). Reading speed exceeds $90\ \text{words/minute}$.
• Unstable Fixation: $< 75\%$ of points fall within $2^\circ$ ($BCEA > 8.0\ \text{deg}^2$). Fixation jumps erratically across the scotoma edge, reducing reading speed to $< 20\ \text{words/minute}$.

Clinical rehabilitation trains patients to intentionally direct their PRL:

1. Audio-Visual Biofeedback (Microperimetry): When the patient aligns their PRL with a target, the device emits an auditory chime, reinforcing cortical oculomotor circuits.
2. Clock-Hour Eccentric Viewing: Patients are taught to deliberately gaze off-center (e.g., 'look at 12 o'clock on the clockface to see the person's face clearly at the center').
3. Within 6 to 8 structured biofeedback sessions, fixation stability tightens significantly, doubling reading speed and maximizing magnification efficacy.