Two classes of tests are used to assess stereoacuity, contour tests and random- dot tests. Contour tests have the disadvantage of providing cues that enable some children with nil stereoacuity to pass the initial 2–4 levels of the tests, typically disparities in the range >100 arcsecs [16, 17]. Large disparity contour targets be identified as different from the other targets on the basis of monocular or nonstereoscopic binocular cues [16, 17]. Most children treated for congenital cataracts will have nil or reduced stereoacuity, so it is especially important to use a random-dot format to distinguish true stereopsis from responses based on other cues in this clinical population. Random-dot tests contain no monocular or nonstereoscopic binocular cues; depth can only be appreciated via a global binocular evaluation of corresponding points and disparate points.

Most stereoacuity tests are conducted at a near viewing distance, usually at 40 cm. Because stereoacuity is sensitive to blur, it is important to optically correct for the viewing distance. It is always best to perform stereoacuity testing early in the examination, before the child’s eyes are dissociated for visual acuity and ocular alignment assessments. For infants and pre-verbal children, forced-choice preferential looking tests can be performed using the PASS Stereotest (“Preschool Assessment of Stereopsis with a Smile” Stereotest; Vision Assessment Corporation, Elk Grove Village, IL, USA). The test set includes 6 vectographic random dot cards that the child can view while wearing polarized glasses. The test begins by holding the demo card with a two-dimensional smiley face side-by-side with the blank card. The child will look at and/or point to the card with the face. Once the child and tester are comfortable with the demo card, the test proceeds in the same way with the other four cards, holding them side-by-side, either to the left or the right of the blank card to determine whether the child consistently prefers to look at or point to the random dot smiley face. The four cards present four levels of stereoacuity: 440, 240, 120, and 60 arcsec. The smallest level of disparity that elicits consistent looking or pointing behavior provides an estimate of the child’s stereoacuity. Normative data are summarized in Table 21.1 [9–15].

For children age 3 years and older, stereoacuity can be tested with matching and pointing tasks, and as they become more verbal, by asking them to name shapes or indicate locations. In this age range, the Randot® Preschool, TNO, and Frisby Stereoacuity tests have become widely accepted standards for random-dot stereoacuity measurement. The Randot® Preschool Stereoacuity Test (Stereo Optical Co, Inc., Chicago, IL) was designed as a matching game in which the patient matches black-and-white pictures on the left side of a booklet with random-dot stereogram pictures on the right. The child wears polarizing glasses to view the stereograms. All of the stereogram pictures are age appropriate for children age 3 years and up (e.g., heart, car, duck, tree). Six levels of disparity are available in the standard set: 800, 400, 200, 100, 60, and 40 arcsecs. An optional supplemental book provides 30 and 20 arcsecs. The TNO (Lameris Ootech, Ede, Netherlands) stereoacuity test plates show discs with a missing sector and the child is asked to “point to where the piece of cake is missing.” Plates V–VII are used for stereoacuity testing and present disparities of 480, 240, 120, 60, 30 and 15 arcsec. The Frisby Stereo Test (Stereotest Ltd, Sheffield, UK) presents “real depth” objects viewed with natural vision; no glasses are required. The test is comprised of 3 plastic plates of different thicknesses. Each plastic plate has four random-pattern squares printed on one side, but one of the squares has a central circle of pattern elements printed on the other side, so that it lies in a different depth plane relative to its surrounding square. The child must identify which of the four squares contains the circle. Using all three plates at varying viewing distances of 30–80 cm, it is possible to test stereoacuity in the range of 600 to 15 arcsecs. Normative data for each of these tests have been published (Table 21.1) but in some cases it is also important to ensure the normative values being used are for the correct version of the test, as variations may be present [18].

Despite the difficulties associated with interpretation of performance on the Titmus Fly, Titmus Animals, and the first 4 levels of the Titmus circles and Randot circles, results from these stereoacuity tests are commonly reported as outcomes for children treated for congenital cataracts. Passing the Titmus Fly test without stereopsis may be possible because the child knows it is a fly and expects the wings to be elevated, because the child has been repeatedly exposed to the same test with only two possible responses, or because the child alternates fixation and observes image jump in disparate portions of the test. Similarly, the location of the disparate circle in the first 4 levels of the Titmus and Randot circles and animal tests can be easily detected by ether monocular or nonstereoscopic binocular cues [16, 17]. Thus, we will include results obtained with these contour stereoacuity tests here with the caveat that stereoacuity values of 140 arcsecs or worse may be the result of artifact rather than evidence of true stereopsis.

Overall, the evidence regarding the prevalence of stereoacuity in children with congenital cataracts is necessarily limited by design characteristics of the currently available stereoacuity tests [19]. While high-grade stereoacuity is unlikely, children treated for congenital cataracts may have stereoacuity that simply is not measurable due to the limited range of disparities available. A second factor that may limit performance on current random dot stereoacuity tests is dot size; most random dot tests require visual acuity of 0.4 logMAR or better in order to be able to appreciate the dot texture sufficiently to extract disparity information. In addition, the clinical tests consist of relatively small images that are static; there is some evidence to suggest that larger, dynamic stimuli provide additional information that may allow children who appear to have nil stereoacuity on standardized tests to appreciate depth [20, 21].

The sudden post-natal onset of stereopsis exhibited by infants at 3 months of age corresponds to the start of a critical period, during which binocular function can be severely and permanently disrupted by abnormal visual experience. This maturational time course is consistent with a model of visual development that posits a pre-critical period, guided by genetically programmed molecular and neuronal signals, and a critical period during which environmental signals maintain and refine the neuronal circuitry for stereopsis. This model grew from Hubel and Wiesel’s series of simple but elegant experiments in which they showed that closing one eye during early visual development permanently altered the structure of the columnar structure of the visual cortex, shifting the ocular dominance of cortical cells to the non-deprived eye [22, 23]. They hypothesized that visual experience guides the development of ocular dominance columns during a critical period shortly, but not immediately, after birth. During this period of experience- dependent maturation, neuronal connectivity and function is highly susceptible to disruption by visual deprivation.

However, this simple model of the critical period is incomplete. Cortical ocular dominance columns form very early, with at least a proto-map present prenatally [24, 25]. The development of neural circuitry in the primary visual cortex to support topographic maps and orientation selectivity is not dependent on visual experience, but is instead guided by molecular signaling and spontaneous neural activity. However, during the critical period, visual experience guides further refinement of the selective properties of striate neurons to harmonize inputs from the two eyes (Fig. 21.2). Prior to the critical period, striate neurons are commonly selective for different orientations when driven by the left versus the right eye [26, 27]. With normal visual experience during the critical period, striate neurons gradually shift their tuning to respond maximally to the same orientation regardless of which eye is viewing. Monocular or binocular visual deprivation during the critical period disrupts this developmental matching process and results in persistent mismatch and loss of binocular excitatory neurons [26, 27].

Classically, the critical period for ocular dominance plasticity has been considered to end at the onset of adolescence [28, 29]. More recent evidence suggests that sensitivity to visual deprivation actually diminishes very slowly and persists beyond adolescence, as does the potential for rehabilitation [30, 31] and is present even in adult striate cortex [32]. The diminishing potential for rehabilitation is reflected in studies that demonstrate a need for longer, more intense treatment and less stability of the gains in visual function obtained with treatment. It has been proposed that the increase in inhibitory cortical function that is a pronounced characteristic of the critical period [26] may also result in maturation of structural factors that restrict remodeling of circuits and promote closure of the critical period [33].

The key role of surgery during the first weeks of life to minimize the effects of visual deprivation due to congenital cataract on visual acuity development is well established (for review, see Birch and O’Connor [34]). As good visual acuity is a prerequisite for high-grade stereoacuity, it is a reasonable assumption that for a good stereoacuity outcome, early surgery is essential. However, like visual acuity, stereoacuity outcome is dependent on multiple factors, not just the timing of cataract extraction. These factors include the laterality of the cataract, treatment modalities, and post-surgical complications.

Unilateral congenital cataract presents multiple barriers to the development of stereoacuity that may be present, both prior to cataract extraction and post-extraction. Prior to extraction, a unilateral cataract deprives the developing visual system of an important excitatory stimulus and also results in a severe imbalance in developing suppressive interocular interactions. Early surgery (≤8 weeks of age) is known to minimize the impact of deprivation and suppression on the development of visual acuity and appears to be of some benefit for stereoacuity development as well. Thus, children who are able to achieve visual acuity outcomes of 0.6 logMAR or better (20/80 or better) are much more likely to pass the various stereoacuity tests than those with visual acuity outcomes worse than 0.6 logMAR (20/100 or worse) (Table 21.2) [35–46].