The lens paradigm in experimental myopia: oculomotor, optical and neurophysiological considerations.

The lens-rearing paradigm has developed great importance in the field of experimental myopia. Although an apparently simple paradigm, the results of any experiment can be influenced by a variety of factors including habitual viewing distance, ocular refraction, oculomotor performance and the spatial sensitivity of the retinal elements involved in retinal image assessment. Computer modelling has been used to evaluate the expected impact when lenses are placed in front of a primate eye as a function of the above parameters. Spatial band-pass responses of the mechanisms responsible for emmetropization are predicted to lead to a limited range of retinal defocus over which compensation to lenses could occur. Even assuming an equal ability to detect hyperopic and myopic defocus, it is predicted that primate eyes should be able to compensate for a much larger range of minus lenses than plus lenses. This derives from the ability of the accommodation system to keep retinal defocus within this operating range over a wider range of minus lenses than plus lenses. The range over which compensation can occur will depend on the spatial tuning of the elements responsible for detecting retinal defocus and capabilities of the accommodation system. The observed asymmetry in responses of the primate eye to rearing with plus and minus lenses and observed differences between primates and chickens in lens rearing studies may therefore be partly attributable to optical and neurophysiological considerations.