The phrase a resident hears on ward rounds more often than any other: "after seven it's useless". It is convenient because it sounds like a threshold. It is wrong as a threshold — and right as a reminder that time is not free.
The term critical period entered the vision literature after Wiesel and Hubel's work on the ocular dominance of V1 cells in the cat. In the cat, sensitivity to closure of one eye begins at about the fourth week, is high until weeks 6–8, then falls and disappears at about the end of the third month (Hubel, Wiesel, 1970 — the definition cited by the subsequent neurobiological literature). Transferring these weeks onto human years as a table of "critical period up to N" is a species error. The species is different, the function is not single, and in humans even acuity and global motion have different calendars.
Maurer and Lewis (2005, Developmental Psychobiology), studying children deprived of early form vision by dense cataracts, formulate not one period but several sensitive periods. Within acuity they distinguish at least three stretches:
- the period of normal visually driven development — the time when input is needed for acuity to grow to adult levels at all;
- the period of vulnerability to damage — the time when abnormal input can spoil the function, sometimes even after it has outwardly "matured";
- the period of recovery — the time when, after input is restored, it is still possible to catch up.
Moreover, different visual functions have different windows. In a review from the same line of work (Maurer), the estimate of the end of the period during which deprivation can damage acuity reaches approximately 10 years; for global motion the window of damage is described as less than the first year. These are data from people with cataract deprivation, not an extrapolation from the cat and not a promise that a ten-year-old with anisometropia is "still like an infant".
For the clinician the PPP compresses this into a cruder but workable scheme. Untreated or inadequately treated amblyopia, with rare exceptions, produces lifelong loss. The potential for successful treatment is greatest in younger children, but studies included in the PPP show that treatment in older children can improve acuity. A separate hard boundary: untreated deprivation due to significant media opacities in the first 3 postnatal months produces a profound and permanent reduction in high-contrast acuity, typically to 20/200 or worse. Deprivation after 3 months may be less profound. Even brief deprivation in infancy can cause amblyopia. Early cataract deprivation is strongly associated with sensory nystagmus in bilateral cases and with strabismus — in unilateral and bilateral cases.
For a visually threatening unilateral cataract in newborns the PPP gives a benchmark that must not be blurred: the prognosis is better if the cataract is removed and optical correction is in place by 2 months of life (PPP references 14–16, the Birch / Infant Aphakia line of work). This is not "the critical period of amblyopia in general". It is the calendar of the most severe unilateral form deprivation.
Levi (2005) separately examines adults with amblyopia and perceptual learning: adults can improve their performance on a difficult visual task with practice, and part of this improvement may transfer to acuity. This is a research fact about plasticity, not a standard of treatment and not grounds for promising an adult "recovery as at age three". The course keeps this boundary explicit: plasticity is not zero after childhood; a clinically meaningful protocol, reproducible in large RCTs, to "cure adult amblyopia with a course of games" is not among the foundations of this course.
How to talk to the family without lying and without closing the door.
- "The earlier the factor is removed and treatment started, the higher the chance" — true, and consistent with the PPP.
- "After seven nothing can be done" — false. PPP: all children with amblyopia should be offered an attempt at treatment regardless of age, including older children and adolescents, especially those not previously treated.
- "An adolescent responds the same as a three-year-old" — also false. In ATS3 (PEDIG, children aged 7–<18 years, 507 participants, unilateral strabismic or anisometropic amblyopia), in the 7–12-year group the proportion responding to augmented treatment (optics + occlusion ± atropine) was roughly twice that with optics alone (53% vs 25%). In the 13–17-year group the proportions responding to augmented treatment and to optics alone did not differ (25% and 23%). Age changes the probability, not the right to an attempt.
- "There is one critical period, and it is about acuity" — too crude. Acuity, stereopsis, global motion and resistance to deprivation have different calendars.
Summary. The critical period is a family of windows, not a birthday after which the card is closed. The clinician counts the calendar of the cause (a cataract in the first months ≠ moderate anisometropia at six) and does not substitute the school myth about seven for it.