Abstract

A model of rod vision in human infants is proposed in which rod-initiated signals saturate after the rods themselves but before most of the extensive spatial pooling of visual signals that occurs in infants. This model is based on an experiment in which psychophysical absolute thresholds were measured on two month olds using 10-ms short-wavelength test flashes that were 6–16° in diameter. Psychometric functions were steep for large stimuli and shallow for small stimuli. According to the model, rod-initiated signals from large stimuli are pooled over a large retinal region, which overcomes the extreme insensitivity of the proximal pathways to some extent, allowing 100% correct performance to be achieved based on rod-initiated signals. For small stimuli, however, the distal rod pathways of the infant are taxed beyond their capacity to respond; the result is saturation of rod-initiated signals at remarkably low test luminances. The psychometric functions are shallow for small stimuli because 100% correct performance is not attained until the test flash has high enough luminance to be detected by means of cone-initiated signals. Increment threshold data were also collected and are consistent with the model.

© 1988 Optical Society of America