(1) Quantitative visual acuity testing of infants is difficult, if not impossible, in an office environment, and it is usually unnecessary.  Of greater importance is whether there is a difference in visual acuity between the two eyes.

(2) This technique is based on the fact that an infant's attention is attracted more by patterned stimuli than by homogenous surface, provided the pattern is above the visual acuity threshold.  The infant's eye or head movements in response to the appearance of the striped stimulus is interpreted as "seeing" the target.  To adapt this time-consuming method to a clinical setting, acuity cards have been designed that contain grating patterns of varying spatial frequencies.^{15, 38}  The usefulness of this method as an office procedure is still being evaluated.

(3) The fixation behavior is assessed by observation of the eyes while the patient fixates on a visual object or a fixation light.  Searching, unsteady, or nystagmoid fixation movements and inability to pursue a moving visual target indicate poor vision.  The examiner notes whether the child alternates, prefers fixation with one eye, or can hold fixation well with either eye.  For instance, refusal of the child to have the right eye covered while not objecting to having the left eye covered is interpreted as evidence for reduced visual acuity in the left eye.

(4) OKN responses depend on visual acuity, attention, and intact motor responses.  At best, this test provides information regarding whether vision is present.  OKN testing has not been adapted successfully for quantitative testing.^{25, p. 4}

(5) If a patient with strabismus shows random alternating fixation, it is justified to conclude that visual acuity is equal in both eyes.

(6) Fixation preference for one eye usually indicates reduced vision in the non-preferred eye.  However, this conclusion cannot be automatically made because some patients have normal and equal visual acuity in each eye despite a strong fixation preference. 

(7) In cases of fixation preference, subtle differences in the steadiness of fixation and the ability to maintain fixation through a blink are noted. 

(8) Alternation and fixation preference are difficult to judge in the absence of strabismus.  In such instances a vertical strabismus is induced by holding a 10 diopter base-up or base-down before one eye.⁷⁶  A vertical prism held in front of one eye produces vertical dissociation, thereby allowing detection of fixation preference when no or only very small horizontal deviation is found.⁷⁶