Simulations of ideal performance in infant monkeys predict a postnatal improvement in contrast sensitivity at all spatial frequencies, due to the optical and retinal changes given in
Table 1. At spatial frequencies below 8 c/deg, the changes are almost entirely due to changes in cone sensitivity; only at 16 c/deg, the highest spatial frequency we simulated, do the sensitivity values also reflect changes in optical contrast transfer.
Figure 1 plots threshold contrast as a function of spatial frequency for the infant monkey ideal observer (red) and for a single, precocious real observer (green). The curves computed for each age of the ideal observer show the expected form, with a pure low-pass character whose high frequency fall-off reflects the decline in optical transfer with increasing frequency (
Williams and Boothe, 1981). Unlike the simulations of
Banks et al. (1987), these curves are shallower than those measured behaviorally — this is because our ideal observer used stimuli of constant size, while Banks et al. scaled stimulus area as the inverse of squared spatial frequency. The increase in sensitivity with age is quite modest, and takes place almost entirely before the age of 4 weeks, as expected from the values given
Table 1. Note that even though the distribution of foveal cones changes quite dramatically after 4 weeks, that does not change the ideal observer’s performance except by increasing retinal coverage and therefore the fraction of incident light captured. These changes in retinal cone density also substantially sharpen the peak of the retinal cone density function. In infant monkeys, this function is relatively flat, but it sharpens with age as the cones migrate toward the center of the fovea (
Packer et al., 1990). We measured this function in our retinas. From the central fovea to an eccentricity of 1.5 deg, the edge of our 3 deg test targets, linear cone density falls by less than 2% in the 1-week-old animals and by about 10% in the 4-week-olds, and by 45% in the adult (cf.
Packer et al., 1990;
Wikler et al., 1990). Our ideal observer simulations took cone density to be constant across the test field, which is close to the truth for the younger animals. Even in the adult, any errors introduced by deviations from uniformity are negligible, since the ideal observer’s performance is not limited by cone density but by retinal coverage and cone efficiency (
Table 1).