Scotopic center surround organization in 10-week-old infants

The large critical areas for complete spatial summation in infants may be due to immaturities of receptive field organization in both dark and light adapted scotopic conditions. Scotopic lateral excitatory/inhibitory interactions have been previously studied in adults using Westheimer's [(1965) Journal of Physiology, London, 181, 881-894] paradigm. We modified this paradigm to investigate scotopic receptive field organization in infants. Detection thresholds for probe flashes of constant diameter and duration presented on steady backgrounds of various diameters were determined for 10-week-old infants and adults. Infants' thresholds increased up to background diameters of 3 degrees to 3.5 degrees and then decreased at greater background diameters. Adults' thresholds increased for backgrounds up to about 0.75 degrees diameter and then decreased at greater background diameters. These results suggest that the excitatory, central receptive field size is larger in infants than adults, and that a balanced center-surround mechanism is operative.