The critical period: the years vision is built in

Whenever lazy eye comes up, age enters the conversation sooner or later. The reason is that the visual system does not arrive complete at birth.

The eyes collect light from day one, but the brain connections that make sense of that light are built over years and according to the image that arrives. That construction phase is called the critical period.

The answer comes from one of the best-known experiments in vision research. Wiesel and Hubel examined what happens when one eye is deprived of its image early in development, by recording from individual cells in the visual cortex (Wiesel and Hubel, Journal of Neurophysiology, 1963).

The finding was striking: the number of cells responding to the deprived eye had fallen sharply. The eye itself was intact. What had changed was that eye's representation in the cortex.

The decisive detail was timing. The same deprivation applied outside the developmental window did not produce the same effect. The problem was not deprivation as such but when it occurred.

This work is why amblyopia is treated as a developmental problem rather than a disease of the eye.

There is no single date. Different visual functions run on different timetables, and those timetables overlap.

Basic acuity is shaped earliest, which is why anything that blocks the image in infancy, congenital cataract for instance, carries the heaviest consequences and calls for the fastest intervention.

Functions that depend on the two eyes working together, depth perception above all, also have an early sensitive window. That explains the weight placed on noticing a turned eye early.

Sensitivity does not switch off like a light. It tapers gradually, so naming a hard age cut-off would make the phenomenon look sharper than it is.

The common account is that after a certain age nothing more can be done. That is a stronger claim than the evidence supports.

What declines is not the existence of plasticity but how easily it is obtained. The same change requires more repetition, more time and a more targeted stimulus.

Gains have in fact been reported in adults with amblyopia: dichoptic approaches that exercise both eyes together have shown that the adult brain can respond to this kind of training (Li et al., Current Biology, 2013).

The accurate framing is this: the early years are the most favourable window, not the only one.

First, the screening schedule has a real justification behind it. Amblyopia caught early usually resolves faster and with less effort.

Second, a late diagnosis does not mean treatment is not worth attempting. Age is information used to set expectations and duration, not to rule treatment out.

Third, plasticity requires repetition. Short, regular sessions sustained over months are worth more than a plan that is perfect in theory and never carried out. Whether a method can actually be kept up is one of its most important properties.