Achromatic contrast effects in infants: adults and 4-month-old infants show similar deviations from Wallach's ratio rule.

When adults view a disk of light embedded in a higher luminance surround, the perceived lightness of the disk is largely determined by the surround to disk luminance ratio (Wallach's ratio rule). In the present study, both adult and infant subjects were tested with multiple discrete trial procedures in which the surround luminance was decreased between the study and test phases of each trial. Tested with sequential lightness matching, adult subjects showed an approximate ratio rule, with a small but consistent deviation in the direction of a luminance match. Tested with a forced-choice novelty preference technique in combination with a cross-familiarization paradigm, 4-month-old infants showed preference minima that fell closer to the mean adult match than to the ratio rule. This finding suggests that, at least for a relatively simple visual display, 4-month-old infants' looking preferences are governed by an adult-like achromatic contrast system.