Abstract
Abstract: :
Purpose: Chicks' eyes rapidly compensate for defocus imposed by spectacle lenses by changing ocular length and choroidal thickness. Because positive and negative spectacle–lens–compensation is not simply a function of the total amount of lens–wear, but is improved by more frequent episodes of lens–wear (Winawer & Wallman, 2002), we hypothesize that the lens–wear causes a signal to rise and eventually saturate during episodes of lens–wear and to decline slowly between episodes. Methods: We cannot measure a response after only a few minutes of lens–wear. Instead, to estimate how quickly the hypothesized signal rises, we gave frequent (hourly) episodes of lens–wearwear to minimize the decline of the signal between episodes and measured how long the episodes have to be to give half the effect of continuous lens–wear over 3 days (rise–time). To measure the decay of the signal between episodes of lens–wear, we gave long (30 min) episodes of lens–wear to maximize the signal during each episode and determined how long the interval between episodes had to be to again give half the effect of continuous lens–wear (fall–time). In all cases we gave multiple episodes of monocular +7 or –7 D lens–wear during the 14 hr light phase of the day (while keeping the birds in dark between episodes). Results: By comparing the ocular and choroidal compensation induced by various lens–wearing paradigms, we estimated the approximate rise– and fall–time for ocular and choroidal responses (see table). Conclusions: The effect of positive–lenses on ocular elongation has a faster rise–time and and slower fall–time than on choroidal thickness. For negative–lenses, the reverse is true. Thus, negative and positive lenses exert their effects over very different periods, and ocular and choroidal responses appear to be regulated by different signals. These results emphasize that the temporal pattern of lens–wear is as important as its duration.
Keywords: emmetropization • choroid • myopia