This study explores the reason why very young infants are unable to respond differentially to faces and the cause for developmental changes in infant face perception by age 3 months. Linear systems analysis (LSA) and the contrast sensitivity function (CSF) were used to estimate the facial pattern information available to 1- and 3-month-old infants. Two hypotheses were examined: young infants are incapable of selectively attending to and utilizing perceptual information picked up from particular aspects of the face; early performance may be significantly limited by insufficient acuity and contrast sensitivity for picking up critical pattern information. The second hypothesis was evaluated by estimating the extent to which acuity and contrast sensitivity limitations constrain available facial pattern information; and by considering whether sensory constraints accounted for the infant's observed performance. (Author/DB)