Stereopsis refers to binocular vision, which results from the brain integrating images from both eyes to form one image. As a result of stereopsis humans have depth perception. If one eye is not functioning normally or lacks normal vision, the brain may not interpret images from that eye. The resulting visual input lacks depth. Approximately 10 percent of the population lacks stereopsis. Treatment to restore stereopsis includes addressing the underlying cause and training the eyes to work together through vision therapy. Younger patients may be able to gain stereopsis, whereas older patients have lower rates of success.
Knowledge of the concept of stereopsis dates back to artists such as Leonardo da Vinci (1452—1519) who struggled to render the three-dimensional (3D) world on a two-dimensional canvas. Yet stereopsis was not defined until the mid-1800s when Charles Wheatstone published a paper on binocular vision and later created a stereoscope for observing two-dimensional pictures in 3D.
Understanding the concept of stereopsis has opened the door to many types of entertainment in the past century. By the late 1800s many homes had a stereo viewer and sets of photos called stereographs were widely available. In the 1920s, 3D movies were created. View-Master, familiar to many children, allows the viewer to experience 3D scenes including national monuments, landmarks, and animals. In the 1990s, Magic Eye pictures (stereograms that do not require a stereoscope) became popular and allowed viewers to see 3D images without a stereoscope.
Tests for Stereopsis
A variety of tests are used to measure stereoacuity; most of these tests rely on stereograms that can be viewed with polarized glasses. With polarized glasses, a person with stereoacuity will see a 3D image floating above a field of random dots. For children, a picture of a fly is viewed wearing polarized lenses. If the child has stereovision, the fly’s wings will appear to be above the image. For the test, the child will be asked to pinch the fly’s wings. A child with stereopsis will attempt to grab the wings above the image, whereas a child lacking stereopsis will touch the image itself. More detailed tests have been designed for adults that can measure the degree of stereoacuity a person has.
Significance of Stereopsis
Many people who lack stereopsis have amblyopia (lazy eye), strabismus (crossed eye), lack vision in one eye, or have another reason why their eyes are unable to work together. Lack of stereopsis causes a lack of 3D vision and may make sports, some manual tasks, and driving more difficult. Vision training can help a person learn to succeed at any of these tasks. Wiley Post (1898—1935) was the first pilot to fly around the world solo, and since he had only one eye, he lacked stereopsis. Despite a lack of stereopsis he was able to undergo vision training and successfully take off, fly, and land a plane (Elshatory & Siatkowski, 2014).
There may, however, be an advantage for the estimated 10 percent or more of the population lacking stereopsis. Blakeslee (2011) tested art students and compared them to other students and discovered artists have poorer stereopsis. Furthermore, the same study discovered that based on photographs of established artists, a higher percentage of artists have misaligned eyes than persons with “normal” vision (Blakeslee, 2011).
Treatment is focused on restoring stereopsis. In order to have stereopsis a patient must have two eyes with similar visual acuity that are in alignment, transmitting images to the brain in a way that the images can be interpreted. Treatment, therefore, may involve wearing eyeglasses to balance the visual input being sent to the brain, having surgery to align the eyes, or patching the stronger eye to allow the brain to interpret input from the weaker eye. Additional types of vision therapy including eye exercises may restore stereopsis.
If a lack of stereopsis is discovered early and the underlying cause is treated, stereopsis can be restored. There may be a critical age range during which stereopsis can be recovered, but this concept has been challenged by the reports of a neuroscientist dubbed “Stereo Sue” who was able to regain stereopsis in her 50s (Sacks, 2006). Stereo Sue’s story has caused speculation that the adult brain is more plastic than once thought.
Future studies will help us understand the complex mechanism of stereopsis. Further understanding of the adult brain may reveal techniques that allow people who lack stereopsis to regain stereopsis later in life.
Lisa A. Rabe
See also: Amblyopia; Blindness; Optic Nerve Hypoplasia; Strabismus
Blakeslee, Sandra. (2011, June 14). A defect that may lead to a masterpiece. New York Times, p. D6. Retrieved from http://www.nytimes.com/2011/06/14/health/views/14vision.html?_r=2&
Elshatory, Yasser M., & R. Michael Siatkowski. (2014). Wiley Post, around the world with no stereopsis. Survey of Ophthalmology, 59(3), 365—372. Retrieved from http://dx.doi.org/10.1016/j.survophthal.2013.08.001
Sacks, Oliver. (2006, June 19). Stereo Sue. Why two eyes are better than one. The New Yorker. Retrieved from http://www.newyorker.com/magazine/2006/06/19/stereo-sue