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Abstract
During development, the eye grows under visual feedback control, as shown by its compensating
for defocus imposed by spectacle lenses. Under normal conditions the sign and magnitude
of defocus vary with viewing distance, accommodative status and other factors. To
explore how periods of myopic and hyperopic defocus are integrated over time we presented
rapidly alternating episodes of myopic and hyperopic defocus by sequentially illuminating
a nearby scrim and the wall beyond it to chick eyes wearing lenses that put the far
point between the two surfaces. We found that equal periods of myopic and hyperopic
defocus generally led to compensatory hyperopia, showing that myopic defocus had a
disproportionate effect. Furthermore, the degree of hyperopia depended on the frequency
of alternation: low frequencies (1 cycle/30 min) resulted in more hyperopia, whereas
at high frequencies (1 cycle/s) the myopic and hyperopic defocus nearly cancelled
each other. If similar temporal integration effects apply to humans, they may help
explain why brief accommodation events may not influence lens-compensation and why
a child's total reading time may be a poor predictor of myopic progression.