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Peripheral and Central Factors Limiting Spatial Contrast Sensitivity in Infant Macaque Monkeys

To evaluate the contribution of peripheral and central factors to the development of visual sensitivity, we measured contrast thresholds for sinusoidal gratings (0.5 – 4.2 cycles deg−1), presented alone and in the presence of added dynamic visual noise, in infant monkeys. We wished to use the changes in contrast threshold during development, in the presence and absence of masking noise, to establish what mechanisms might be responsible for this developmental process. We tested Macaca nemestrina monkeys, aged 1 – 18 months, on a spatial contrast detection task, and analysed the masking functions to calculate two parameters: equivalent intrinsic noise (Neq, the noise contrast at which squared contrast threshold was doubled), and signal-to-noise ratio (Rsn, the asymptotic ratio of threshold to noise contrast at high noise contrasts). We hypothesise, following Pelli, that Neq measures a peripheral limit while Rsn measures a central one. Our data suggest that intrinsic noise is high in infants and falls with contrast threshold during development, thus supporting the hypothesis that peripheral factors set an important limit on the development of contrast sensitivity. However, the relationship between Neq and contrast threshold varied somewhat with spatial frequency. At low spatial frequencies, contrast threshold was well specified by knowledge of Neq, whereas at higher spatial frequencies, changes in Rsn had a relatively greater weight. We conclude that peripheral factors set an important limit on the development of sensitivity at all spatial frequencies; central factors are needed to account for some aspects of development at higher frequencies.