We examine the contributions of preneural mechanisms, i.e., the optics of the eye and the aperture, spacing, and efficiency of foveal cones, to poor spatial and chromatic vision in human neonates. We do so by comparing the performances of ideal observers incorporating the characteristics of the optics and the foveal cones of adults and neonates. Our analyses show that many, but not all, of the differences between neonatal and adult contrast sensitivities and grating acuities can be explained by

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... age-related changes in these factors. The analyses also predict differing growth curves for vernier and grating acuities. Finally, we demonstrate that preneural mechanisms constrain chromatic discrimination in human neonates and that discrimination failures may reflect poor visual efficiency rather than immature chromatic mechanisms per se. Four optical properties are important to the derivation of an ideal observer for spatial and chromatic visual tasks: posterior nodal distance, pupil size, ocular media transmittance, and the optical transfer function. As shown in Table 1 , we used posterior nodal distances of 11.7 and 16.7 mm for newborns and adults, respectively.' 2 -17 Image magnification-is proportional to posterior nodal distance, so the adult eye is