The effect of light adaptation on scotopic spatial summation in 10-week-old infants.

Psychophysical area-intensity functions of individual 10-week-old human infants and adults were obtained in the dark adapted state, and in the presence of a steady background that elevated threshold 1 log unit above the dark adapted level. For dark adapted infants, the mean diameter for complete spatial summation (4.42 degrees; SD: 1.67 degrees) was significantly larger than that of adults (2.32 degrees; SD: 0.09 degrees). The background reduced the mean critical diameter to 2.67 degrees for infants (SD: 0.64 degrees) and to 1.16 degrees for adults (SD: 0.08 degrees). Spatial probability summation has similar effects on infant and adult thresholds, and, therefore, does not appear to account for the developmental decrease in critical diameters. Rather, decreases in receptive field size are suspected.