One of the first questions people ask when the find out that an optometrist is going to do a vision exam on an infant is, "How can you tell what they can see?"  What they are familiar with as adults is a Snellen letter chart projected at the end of the examination room from which they are asked to read the smallest letters they are capable of seeing.  They know that an infant is not capable of performing that task, so they are puzzled as to how the task can be modified so that an infant could give reliable results on this type of test, a test of visual acuity (the sharpness and clarity of ones vision).

        Examiners of infant vision have benefited from a technique originally discovered, as many great discoveries are, almost by accident in research on what infants would look at and what they would not.  A researcher, Robert Fantz, discovered that infants had definite preferences for what they would look at or fixate on when given choices, and that these preferences were repeatable for individual infants and consistent across several infants.  This led, after many years of intense research effort by many individuals, to the development of a clinical test based on the fixation preferences of infants.  Infants would naturally, and from birth, prefer to fixate a formed target as opposed to an unformed or blank one.  For purposes of visual acuity testing using what is called this Preferential Looking technique, infants are shown a large card with a pattern, in this case black and white stripes, on one side and an equally bright gray pattern on the other side.  As long as the infant can see the separations in the black and white stripes such that they look like a pattern, the infant will invariably prefer to look at the pattern.  What is observed is an immediate and consistent preference for looking at the patterned side of the card when it is shown to the child.

      Cards with stripes of varying widths are used for the clinical testing.  When the stripes are so narrow that the infant can no longer see them, then they look no different that the matching gray side, and the infant no longer has a visual preference for one side or the other.  At this point, the testing is finished.  The narrowest stripe width that the infant showed a preference for is an indication of their visual acuity.