Sight Acquisition After Treatment for Congenital Blindness

Congenitally blind people who acquire sight still need to learn to see. Key points

• Visual areas of the brain typically develop through visual experiences in early childhood.
• People who acquire sight after early childhood can still develop many important visual abilities.
• Brain regions linked to higher-order visual functions are modifiable even in adults.

S. R. D., a 34-year-old woman from the Indian state of Gujarat, was born with cataracts in both eyes (Ostrovsky et al., 2006). She perceived only light until she was 12 years old, when a surgeon removed her cataracts. Eyeglasses increased her visual acuity to 20/200, which is equivalent to seeing the largest letter on an eye chart.

Although she could now see objects, she had trouble understanding what she was seeing. Her childhood blindness affected the development of brain areas responsible for pattern vision. “Pattern vision” is the set of visual processes that combine perceptions of an object into a coherent and distinct whole. Without pattern vision, our visual experiences become a chaotic parade of disjointed sensations.

Clinicians have often assumed that unless blindness is reversed before the end of early childhood (about six years of age), a permanent deficit in pattern vision will occur (Ostrovsky et al., 2009). Their assumption was based, in part, on the results of experimental research on visual development in the domestic cat.

Three Blind Cats

In the 1960s, two Harvard neurobiologists, David Hubel and Torsten Wiesel, found that depriving kittens of visual experience soon after birth disrupted visual development (Hubel & Wiesel, 1998; Parrey, 2024). This deprivation resulted in permanent blindness under some conditions. They concluded that the normal development of the feline visual system depends on visual experiences during a critical period in the first months of life (Daw, 2009). This result raised the possibility that humans may have similar critical periods.

Children on a Farm by Camille Pissarro, oil on canvas. An impressionist painting of two children sitting on a large patch of grass during the day. There are house-like buildings behind them, along with a couple of background figures and some trees. The painting is primarily done in many splotches of green, yellow, light orange, gray-blue, and tan.

Source: Wikimedia Commons / Camille Pissarro

Experience Catalyzes Sight Acquisition After Blindness

In 2004, Pawan Sinha, a neuroscientist at the Massachusetts Institute of Technology, initiated a research program to test the critical-period theory in humans. The research program was a part of Project Prakash, a charitable effort in India dedicated to treating congenital blindness in young people (McPherson, 2019; Sinha, 2013).

Earlier research had raised questions about the extreme version of the critical-period theory (e.g., Gregory & Wallace, 1963; Valvo, 1971). The research results hinted at the possibility that visual experience may improve visual function in those acquiring sight after childhood.

S. R. D.’s family provided anecdotal accounts of the effectiveness of her post-surgical experiences on object and face recognition. With her mother’s help, “S.R.D. learned to recognize her siblings and parents 6 months after surgery, and after a year could name objects around the house purely by sight.” (Ostrovsky et al., 2006)

In adulthood, she learned to walk more than a mile to the homes of families where she worked as a maid. And she learned to visually recognize the members of these families.

Figure dans un Fauteuil by Pablo Picasso, oil on canvas. A cubist painting of a nude figure, done primarily in shades of blue, gray, and tan. The figure has no distinguishable facial features and is nearly indistinct from the background.

Source: Wikimedia Commons / Pablo Picasso

In their experimental research, Sinha and his colleagues assessed S. R. D.’s ability to recognize objects and perceive faces (Ostrovsky et al., 2006). She performed relevant tasks, such as locating and counting the objects in an image and distinguishing faces from nonfaces. In most tests, S. R. D. performed as well as people who had been sighted all their lives.

Thus, her visual deprivation in childhood did not permanently prevent S. R. D. from learning some higher-order visual abilities. The researchers concluded that, for these abilities, “the visual cortex retains its plasticity [i.e., its modifiability] even across several years of highly compromised visual experience.” (Ostrovsky et al., 2006)

An important part of pattern vision is “visual parsing”: the ability to distinguish separate objects in a visual scene (Reich & Amedi, 2015). Sinha and his colleagues found clear evidence for improved visual parsing after sight acquisition in S. K., who received treatment for congenital blindness at 29 years of age (Ostrovsky et al., 2009).

S. K. was born without lenses, which meant that incoming light could not be focused on his retinas. He had a visual acuity of 20/900. Eyeglasses provided by Project Prakash improved his vision to 20/120.

The researchers asked S. K. to wear the glasses while looking at images containing various objects, such as geometric shapes and human faces. They instructed him to count the number of objects in each image, point to the objects, and try to name them. The images contained either:

• Discrete or overlapping objects
• Moving or stationary objects

S. K. had no trouble identifying geometric shapes as long as they were separated from each other. But he could not accurately parse images when objects overlapped. According to Sinha (2013), the world for S. K.,

An image of two overlapping squares, each at 70% opacity, on a white background. One square is green and the other is orange.

Source: Kiera Feng

was a bewildering collection of many regions of different colors and brightnesses, with little to glue them together into coherent entities. Even simple line drawings, such as a circle overlapping a square, appeared to him to be a set of strangely shaped interlocking pieces.

S. K. had less difficulty parsing overlapping geometric figures when one of them was put in motion. Motion helped him to group together the different parts of an object according to the rule, “things that move together belong together.” (Sinha, 2013)

The researchers continued to give S. K. the same tests every few months. Eventually, he learned to accurately parse the images:

A year and a half after the initial intervention, without any training other than his exposure to the visual world around him, SK made marked improvements. He was now able to correctly parse static images. (Sinha, 2013)

In the same study, the researchers tested two children aged seven and 13 years (Ostrovsky et al., 2009). These children also learned visual-parsing skills through experience, and in less time than it took for S. K., probably because their brains were still maturing.

The researchers concluded that the:

evidence of marked improvement in our subjects’ performance … suggests that visual skills related to the complex task of image parsing can be acquired even after a prolonged delay, although the rate of acquisition slows down with age. (Ostrovsky et al., 2009)

Since its inception, Project Prakash has shown that critical periods are important for deficits in some visual functions, such as visual acuity. But other visual functions, such as visual parsing, can be learned even well into adulthood.

References

Daw, N. W. (2009). The foundations of development and deprivation in the visual system. The Journal of Physiology, 587(12), 2769-2773. https://doi.org/10.1113/jphysiol.2009.170001

Gregory, R. L., & Wallace, J. G. (1963). Recovery from early blindness. Experimental Psychology Society Monograph, 2, 65-129. Retrieved from https://www.richardgregory.org/papers/recovery_blind/2-the-case.htm

Hubel, D. H., & Wiesel, T. N. (1998). Early exploration of the visual cortex. Neuron, 20(3), 401-412. https://doi.org/10.1016/s0896-6273(00)80984-8