In the present study we investigate the dependence of photopic contrast thr
esholds on retinal illuminance in infants and adults. Contrast thresholds w
ere measured at five retinal illuminances between about 6 and about 20 000
Td in subjects in both age groups. The forced-choice preferential looking t
echnique was used in 3-month-old infants, and standard forced-choice techni
ques 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 w
ith increasing retinal illuminance. However, at both ages, above a critical
illuminance of about 200 Td, contrast thresholds remained constant, follow
ing 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 adap
tation, our results imply that infants elevated contrast thresholds cannot
be explained solely on the basis of photoreceptoral immaturities. Later phy
siological immaturities must also limit infants' photopic contrast threshol
ds. (C) 2001 Elsevier Science Ltd. All rights reserved.