Acuity card procedures and the linearity of grating resolution development during the first year of human infants.

Since the early times of preferential looking (PL), improvement of procedures to test visual resolution in infants has reached the point where the technique is spreading to clinical practice as it is applicable to a large range of age with a high degree of reliability. Acuity cards allow estimation of bi- and monocular acuity in a few minutes. Data from the literature, obtained with various PL techniques and procedures are compared to explain some incongruities. Procedures for presentation of the cards and criteria for threshold determination can greatly bias the data when they do not take into account the infant's behavioral requirements. To test the linearity of the development of visual resolution, a series of grating acuity measurements was performed on a population of 11 normal infants, regularly tested during their first year of life in optimal conditions. It was observed that: (1) grating acuity develops very regularly during the first year, (2) the two eyes never differ from each other by more than the smallest difference measurable with the equipment used (one half-octave), (3) binocular acuity appears to slightly exceed monocular acuity. These results stress the need for early detection of deviation from the norm and prompt therapeutic action.