Infant light adaptation shows Weber's law at photopic illuminances

In the present study we investigate the dependence of photopic contrast thresholds on retinal illuminance in infants and adults. Contrast thresholds were measured at five retinal illuminances between about 6 and about 20,000 Td in subjects in both age groups. The forced-choice preferential looking technique was used in 3-month-old infants, and standard forced-choice techniques were used in adults. The stimulus was a 0.25 cy/deg squarewave grating phase alternated at 6 Hz. Infants' contrast thresholds were more than two log units higher than those of adults at all retinal illuminances. Contrast thresholds had a similar dependence on retinal illuminance in both infants and adults. For both age groups, contrast thresholds initially decreased with increasing retinal illuminance. However, at both ages, above a critical illuminance of about 200 Td, contrast thresholds remained constant, following Weber's law. Thus a vertical shift was sufficient to bring the two data sets into correspondence. In the context of a two-site model of light adaptation, our results imply that infants' elevated contrast thresholds cannot be explained solely on the basis of photoreceptoral immaturities. Later physiological immaturities must also limit infants' photopic contrast thresholds.