A Shift in Perception: Exploring Stereovision Recovery

Our ability to perceive the rich, three-dimensional nature of the world depends on stereopsis. It is a process by which the brain combines slightly different images from each eye (binocular disparity) into a single depth-rich view. This function plays a vital role in accurate depth estimation, visuomotor control, and many everyday tasks.

Binocular convergence is when both eyes rotate inward to focus on an object. The closer the object, the larger the angle between the eyes’ lines of sight, giving the brain a depth cue.

Source: Sarune Savickaite, created with Canva

However, not everyone shares the same capacity for stereovision. Around 5 percent of the population is considered stereo-deficient, and in older adults, this figure might rise above 34 percent. While people without stereopsis can still rely on monocular cues (such as perspective and occlusion), binocular disparity is key for fine depth judgments and complex motor tasks. A lack of stereovision can affect activities such as sports, reduce safety in older age, and limit career options for roles such as drivers, pilots, and surgeons.

It has long been believed that stereovision develops early in life (between a few months to 5 years of age) and that outside of this critical period, recovery is not possible. However, there are interesting accounts of improvement in adulthood. In the 19th century, ophthalmologist Louis Emile Javal treated his sister's strabismus using a stereoscope, which led to regained depth perception. More recently, psychologist Bruce Bridgeman reported spontaneously regaining stereovision while watching a 3D film (Hugo) without any additional visual training.

One of the most well-known cases of stereovision recovery is the case of Stereo Sue, as described by neurologist Oliver Sacks. Stereo Sue (Dr. Susan Barry) lived most of her life with strabismus (misalignment of the eyes), leaving her stereo blind. Although she underwent several surgeries as a child, her brain never developed the binocular vision networks necessary for perceived depth through the fusion of two separate eye images. Like many stereo blind individuals, Barry (a neuroscientist) learned about her condition only in college. She realized that the visual world she experienced was very different from the depth others perceived.

At age 28, Stereo Sue began vision therapy that involved eye alignment exercises using prisms, Polaroid filters, and the Brock String technique. She started experiencing flat fusion (a brief alignment of images from both eyes) and then quickly progressed to perceiving depth in everyday objects. Items around her, such as leaves and stones, now seemed to "pop out" with clarity. Barry described her experience as seeing a world only ever imagined, an emotional revelation that questioned her understanding of human vision and critical developmental periods.

The Brock String exercise is a vision therapy tool for training eye coordination and focus. By concentrating on each bead in turn, users get instant feedback on whether their eyes are correctly converging or diverging at different distances, helping treat vergence disorders.

Source: Sarune Savickaite, created with Canva

The story of Stereo Sue challenged the longstanding neuroscience dogma that the ability to develop stereoscopic vision is restricted to a narrow critical period in childhood. Her case provided evidence for adult brain plasticity, showing that with appropriate training, the adult visual system can still create new neural pathways. A rich emotional narrative and Barry's insights can be found in an overview in Nature Neuroscience and a New Yorker article by Oliver Sacks.

These cases have encouraged systematic investigations into adult stereovision recovery. Some research has explored methods like temporary patching of one eye to force reliance on monocular cues, perceptual learning—repetitive practice on depth tasks over many trials, or even virtual reality. Interestingly, dressmakers have been found to have better stereovision than individuals in other professions, possibly due to years of fine visual work, suggesting that some improvement can occur naturally through everyday experiences. Stereovision is primarily processed in the brain rather than the retina. This has led some researchers to question whether current training methods truly restore stereopsis or whether reported improvements might be simply coincidental.

Stereovision differences discussed suggest that our perceptual worlds are shaped not just by biology but by our personal history, environment, and learned skills. What we perceive is the product of our neural wiring and our lived experience. This internal reality is often hidden from others, and sometimes even from ourselves, until the right moment or method reveals it.