Teaching case. Two children with the same amblyopic-eye acuity of 0.3 (decimal). One has a gross stereoscopic response to a contour test; the other shows suppression on the Worth 4-Dot already at near. The same number on the chart does not mean the same binocular fate. A course that teaches only acuity lines produces a resident who cannot explain to a family why "depth" needs to be tested at all.
Binocular vision is not a single property but a set. The PPP lists sensory fusion, stereopsis, fusional vergence (motor fusion) and coordinated binocular eye movements. Sensory fusion is the ability of the cortex to perceive two monocular images as one. Stereopsis is the perception of depth from horizontal disparity. Motor fusion is the vergence movements that keep objects on corresponding areas of the two retinas.
Amblyopia, strabismus, refractive error and deprivation hit these components in different ways. That is why there is no "single binocular test" in the clinic. The Worth 4-Dot assesses sensory fusion (and suppression), the Randot assesses stereopsis, prisms assess fusional reserves. The PPP insists: sensory tests are done before dissociating manoeuvres — before occluding for monocular acuity and before the cover test. Otherwise you yourself have destroyed what you set out to measure.
High-grade stereopsis is associated with normal alignment. This is not proof that stereopsis treats strabismus, nor a promise that "depth will appear" after occlusion. It is a diagnostic anchor: if a child has high-grade stereopsis, a gross constant tropia at that moment is unlikely. The reverse is not symmetrical: absence of stereopsis does not prove amblyopia — it is broken by strabismus without amblyopia, and by not understanding the test.
Why this belongs in a module on mechanism and not only in diagnosis. Because the forms of amblyopia break the input in different ways.
- In strabismic amblyopia the two images cannot be fused: the cortex selects the fixating eye and chronically reduces responsiveness to the non-fixating one. This is competition between non-fusable inputs.
- In anisometropic amblyopia one image is simply less well focused. The PPP holds that both the direct effect of blur on the development of acuity and interocular competition similar, but not necessarily identical, to strabismic competition are at work here.
- In isoametropic bilateral amblyopia both images are blurred approximately equally: there may be no "who is in charge" competition; there is bilateral defocus.
- In deprivation the image is destroyed optically (cataract, ptosis, vitreous haemorrhage). If the deprivation is unilateral, the same competition is added to the degradation.
From this follows a rule that students often turn upside down. Occlusion of the fellow eye is a way to temporarily switch off the competitor so that the amblyopic channel gains access to the cortex. It does not "train the muscles" and does not "make the retina work". It changes the statistics of the input. The price of this change is precisely binocularity for the duration of patching: while the fellow eye is covered there is no stereopsis. Atropine penalisation leaves the peripheral field and part of the binocular experience; this is one of the practical differences, not a moral advantage of the drops.
One more consequence. The goal of treatment in the PPP is formulated as equal acuity in the two eyes, "although it is not always achievable". Equal acuity is not a synonym for full binocular vision. A child can reach 1.0 / 1.0 and retain suppression. A child with slight residual anisometropia can have useful stereopsis with an incomplete line. The outcome cannot be reduced to a single cell in the chart.
The lesson's boundary. The course does not cover the fine classification of anomalies of retinal correspondence and does not teach diagnosis on the synoptophore. For the teaching task it is enough that: amblyopia lives in the cortex; the cortex weighs two inputs; treatment changes those inputs; acuity is the main, but not the only, measure of what has changed.