Saturation of rod-initiated signals in 2-month-old human infants

Absolute and increment thresholds were measured for 2-month-old human infants by using long-wavelength adapting fields and short-wavelength test flashes. The test flashes were 500 or 10 msec in duration and varied from 6 degrees to 16 degrees in diameter. The psychometric functions measured in the dark were steep for large, long-duration test flashes and shallow for small, brief flashes. Under conditions of light adaptation, the psychometric functions were consistently steep. The shape of the area-threshold function depended on the criterial percent correct responses; when a criterion of 75% correct responses was used, the slope of the area-threshold function was much steeper than predicted by Ricco's law. These results are interpreted in the context of a model in which rod-initiated signals saturate at a locus proximal to the rods themselves but distal to most of the considerable spatial pooling of visual signals that occurs in infants.