The way humans acquire and perceive visual input is a complex and fascinating subject. Having two forward-facing eyes separated by a small distance allows each eye to have a slightly different view of the world. Known as binocular disparity, it is the basis of stereoscopic viewing, or depth perception.
Frequently, the optometrist is faced with very subtle findings and is forced to decide whether these represent normal variations or something that warrants an emergent workup and referral. In such cases, the most helpful element of the examination is the presence or absence of a spontaneous venous pulse (SVP).
Greater attention to stereo vision testing can elicit pathology and help guide the course of corrective lens wear.
Clues in the Anatomy
Stereopsis is the result of higher-order visual processing that allows three-dimensional data to be extracted through the comparison of these slightly dissimilar retinal images observed from different vantage points.1,3,4 It allows humans to gauge spatial relationships and is crucial in many task that require depth perception, such as driving, sports, visually guided hand movement, motor control and viewing three-dimensional movies.5 Amblyopia is the most common condition that may affect stereoscopic depth perception under ordinary binocular viewing conditions.5
Amblyopia, a neuro-developmental disorder that manifests within the first three years of life (i.e., during neural plasticity), results from early abnormal visual experiences such as strabismus and refractive errors.5 This condition requires early diagnosis and intervention to preserve the quality of vision.
Recent studies have evaluated the possibility of treatment past the critical period to recover vision and stereoacuity. While researchers previously thought that patients over the age of seven will not benefit from treatment, recent studies show some promise in the induction of plasticity even beyond this critical timing.5
Eye care practitioner must be able to accurately diagnose patients with amblyopia as early as possible to ensure effective and time-sensitive treatment. Since amblyopia is an abnormality of neurons on a cortical level, the clinician must rely on identifying the presence and timing of amblyogenic factors as well as ruling out contributory ocular pathology. Stereopsis testing is a helpful tool in these instances.
Table 1. Stereoacuity Testing Methods
|Lang I||Random dot technique||1200 – 550 seconds of arc|
|Lang II||Random dot tehnique||600 – 200 seconds of arc|
|Titmus||Cross-polarized filters||800 – 100 seconds of arc and 3000 seconds of arc|
|TNO||Red/Green filters||480 – 15 seconds of arc|
|Randot||Polarized vectographs||500 – 20 seconds of arc|
Sizing Up the Options
Stereopsis in amblyopes correlates with the degree of reduction in visual acuity. It is also more diminished in patients with monocular rather than binocular blur, and in patients whose amblyopia is strabismic instead of anisometropic in origin.2,5 Many commercial in-office tests for stereopsis are available.
Lang I: This test card contains illustrations of a cat, star and car, each with varying levels of disparity. The test uses random dot and cylindrical gratings without the use of filters. The disparities range from 1,200 to 550 seconds of arc.6
Lang II: This test is similar to Lang I but contains illustrations of an elephant, car and moon. The range of disparity falls between 600 and 200 seconds of arc.6
Titmus: Unlike the aforementioned tests, the Titmus stereo test requires the use of cross-polarized filters and consists of identifying the elevated circle or animal in a set. The disparities range from 800 to 100 seconds of arc.6 Part of the test includes the Wirt fly, which represents the largest level of disparity available in a commercial test at 3,000.7
TNO: Patients use red/green filters for dissociation and are required to identify hidden objects in a series of plates. This also employs random dot techniques.6,7
Randot: Polarized vectographs are used here to present different images to each eye. The difference is not noticeable to the test taker since humans are not able to appreciate light polarization. As a result, both images appear identical, except for the disparity.5,8
The use of monocular cues is an important consideration in the administration and interpretation of these tests, and can result in false positive stereo results. Monocular patients without stereovision can elicit information on the depth of an object using methods such as recognizing the smaller visual angle an object would subtend, the further away it is in space.1 When implementing the use of chairside stereo tests, clinicians should remember that some shifts in the contours of Randot circles, for example, are visible under monocular viewing conditions.8
Overall, stereo testing is a readily available, quick and cost-effective method to identify and quantify the level of binocularity in a patient. These tests offer valuable information for the diagnosis of amblyopia, as well as the progress of treatment, given that visual acuity and stereovision are directly correlated. Early identification is crucial in the preservation of vision in these patients, and this is a quick and helpful screening tool that we should often consider.
1. Westheimer G. Clinical evaluation of stereopsis. Vision Research. 2013(90):38-42.
2. Nityananda V, Read JC. Stereopsis in animals: evolution, function, and mechanisms. J Exp Biol. 2017;220(14):2502-12.
3. Dattilo D, Vasseneix C, Bruce BB, et al. Correlation between stereopsis and reverse stereopsis. Ophthalmology. 2017;124(3):411-3.
4. Plourde M, Corbeil ME, Faubert J. Effect of age and stereopsis on a multiple object tracking task. PLoS One. 2017;12(12):1-8.
5. Levi DM, Knill DC, Bavelier D. Stereopsis and amblyopia: A mini-review. Vision Res. 2015;114(9):17-30.
6. Ancona C, Stoppani M, Odazio V, et al. Stereo tests as a screening tool for strabismus: which is the best choice? Clinical Ophthalmology. 2014;8:2221-27.
7. O’Connor AR, Tidbury LP. Stereopsis: are we assessing it in enough depth? Clin Exp Optom. 2018;101:485-94.
8. Read JCA. Stereo vision and strabismus. Eye. 2015;29:214-224.