Severe astigmatic blur does not interfere with spectacle lens compensation

PURPOSE. Whether either natural emmetropization or compensation for imposed spectacle lenses requires the visual system to distinguish myopic from hyperopic blur is controversial. Some have argued that the visual system need only respond to the magnitude of the blur. This study was undertaken to test whether adding large amounts of astigmatic image blur would cause myopia and interfere with compensation for positive or negative spectacle lenses. METHODS. Chicks were fitted with mixed astigmatic (toric) lenses with + 5 D on one meridian and - 5 D on the orthogonal meridian (Jackson crossed cylinders), thus producing massive blur while having no spherical equivalent power. Chicks wore these lenses either alone or in combination with + 6 D, + 3 D, - 3 D, or - 6 D spherical lenses. To produce a similar degree of image degradation in a different way, other chicks were fitted with weak diffusers, with or without spherical lenses. RESULTS. Eyes fitted with astigmatic lenses alone became mildly hyperopic, whereas those fitted with weak diffusers became mildly myopic. Eyes wearing both the astigmatic and spherical lenses compensated as completely for the spherical lenses as did the eyes wearing spherical lenses alone-the compensation being caused by changes in both choroidal thickness and ocular length. Eyes wearing weak diffusers and spherical lenses also compensated but did so without a change in choroidal thickness. CONCLUSIONS. Severe astigmatic blur does not interfere with spectacle lens compensation. From this it can be inferred that the average amount of blur is not an important error signal for spectacle lens compensation and therefore probably not for emmetropization either. These results strengthen the case that the sign of defocus is used to direct eye growth.