Differential brightness thresholds in infancy

Differential brightness thresholds in infants were investigated by elicitation of optokinetic nystagmus to a moving field of alternate light- and dark-grey stripes. The contrast between the light and dark stripes was varied on successive trials by changing the brightness of the lighter stripes. The smallest contrast which elicited nystagmic movements of the eyes and the largest contrast which failed to elicit these pursuit movements were recorded for each infant. The differential brightness threshold or Weber fraction was assumed to lie midway between these minimal suprathreshold and maximal subthreshold contrasts. Using this procedure with ten neonates, from 1 to 4 days of age, and 10 older infants from 45 to 113 days of age, the average Weber fractions were .50 and .26, respectively. This difference, significant at the .001 level, was taken as evidence for the rapid development of brightness discrimination in the first months of life.