In a matter of minutes, the eye can know which way to grow.

PURPOSE The fitting of chick eyes with positive or negative lenses causes eye growth to decelerate or accelerate, respectively, thereby minimizing the imposed blur. This study was conducted to determine whether the eye can initially assess the correct direction of growth or whether it relies on trial and error, reversing its direction if the magnitude of blur increases. The rapid changes in choroidal thickness in response to brief periods of defocus were measured. METHODS After their eyes were measured by ultrasound biometry, chicks wore either a +10-D lens over one eye for 10 minutes while restrained in the center of a 60-cm drum (to ensure myopic blur), or a negative lens (-7 or -8.6 D) over one eye for 10 minutes or 1 hour in a normal cage environment. They were then kept in darkness until they were remeasured 2 hours, 1 day, or 2 days after the first measurement. Other chicks wore +10 or -8.6-D lenses briefly and were measured several times over the next 7 hours in darkness. RESULTS Wearing positive or negative lenses for only 10 minutes produced significantly different effects on choroidal thickness measured 2 hours later. Wearing positive lenses for 10 minutes caused an increase in choroidal thickness (in 28 of 32 eyes) and a concomitant decrease in vitreous chamber depth, relative to the amount of change in the untreated fellow eye over the same period. Wearing negative lenses for 1 hour caused significant changes in the opposite direction. Wearing lenses for 2 hours resulted in choroidal changes that persisted in darkness for up to 6 hours after positive lens wear, but returned to normal after negative lens wear. Finally, 1 hour of positive lens wear caused significant inhibition of ocular elongation over the next 2 days. CONCLUSIONS The eyes of chicks require only a brief period of lens wear to initiate compensation in the appropriate direction. Because the refractive status changes little during the period of lens wear, the authors conclude that eyes can rapidly determine the sign of the imposed blur without resorting to a trial-and-error method.