Contrast insensitivity: the critical immaturity in infant visual performance.

This is a targeted review of the critical immaturities limiting psychophysical luminance contrast detection in human infants. Three-month-old infants are 50 times less sensitive to contrast than adults are. Rod experiments suggest that early-stage immaturities, like the short length of infant rod outer segments, have only a modest direct effect on infant visual performance. Infant contrast sensitivity may resemble adult extrafoveal sensitivity, because the foveal cones of the neonate are immature and may not generate strong enough responses to mediate visual performance. This use of the extrafoveal retina reduces the high-spatial frequency end of the infant contrast sensitivity function (CSF), contributing to poor infant resolution acuity. The remaining difference between infant and adult CSFs may be a simple overall reduction in infant sensitivity. The maximum of the infant CSF increases proportionately with age, and may be numerically near the infant's age in weeks. Contrast discrimination experiments indicate that the critical immaturity that limits infant contrast sensitivity is a mid-level phenomenon, occurring before the site of the contrast gain control. For example, the infant ascending visual pathway might be limited by large amounts of intrinsic noise. These results suggest that there is little effect of inattentiveness to the psychophysical task by ostensibly alert infant patients or subjects. The clinician or researcher can interpret behavioral measurements of infant visual performance with confidence.