Abstract
When animals are fitted with spectacle lenses that impose myopia or hyperopia their eye growth compensates for the lenses, quickly restoring their original refractive error. This finding constitutes strong evidence that eye growth is under homeostatic control, with visual signals being used as error signals guiding the rate of elongation of the eye. This conclusion, in turn, raises the question of what is the nature of the visual signal used as an error signal. Because at least some lens-compensation occurs in eyes with optic nerves severed, the analysis of the visual signal appears to take place in the retina. Given the presumed low intelligence of the retina, one is tempted to suppose that some analysis of the quantity of blur is used to guide eye growth. However, the evidence to date favors the view that it is the sign, rather that the magnitude, of blur that is decoded to guide eye growth. Thus chicks can compensate appropriately for positive and negative lenses in the face of massive image degradation caused by weak diffusers or astigmatic blur or highly defocused images. We will discuss, first, what optical error signals might survive such image degradation to guide eye growth and, second, whether the blur that guides emmetropization, the blur that guides accommodation, and perceptual blur are all the same blur.