Vision seekers: giving eyesight to the blind raises questions about how people see

Science News, Nov 22, 2003 by Bruce Bower

One witheringly hot day last summer, a 10-year-old boy performed a few miracles at a hospital near Calcutta, India. For openers, he caught a balled-up piece of paper thrown to him. Then, he picked up paper clips and inserted them into a holder through a small opening. Looking determined, the boy proceeded to identify drawings of an elephant and other animals. Finally, he greeted all of his physicians and nurses, referring to each by name.

Not impressed? These accomplishments sure looked miraculous to Pawan Sinha, a neuroscientist at the Massachusetts Institute of Technology (MIT) who was in Calcutta visiting the hospital. Sinha knew that the boy had had severe cataracts in both eyes since birth. He had grown up in a poor family, and the reason for his blindness went undiagnosed until he tripped and broke his leg at age 10. A physician treating the boy's leg instantly noticed the youngster's cataracts and arranged for free surgery.

Five weeks later, the boy--a new-comer to the world of sight--dazzled Sinha with visual feats. It's not yet clear whether a child deprived of sight for many years can learn to see the world with all the subtlety and skill of a person who grew up with normal vision, however. Researchers are just beginning to piece together how the brain responds to blindness early in life and then how it reacts to the sudden unleashing of vision, however years or even decades later.

What's evident, though, is that sight requires far more than simply opening one's eyes and letting reality in. Perception, whether through vision or any other sense, is an acquired taste. People learn to make visual sense of faces and other items of interest, often during infancy and early childhood but sometimes over much longer periods.

A person's view of the world feeds off his or her past experiences with three--dimensional space, the physical details of particular settings, and the predictable shapes and colors of various items, to name a few.

When the loss of sight deprives young eyes of visual experience, other faculties fill the void: Brain regions traditionally thought to handle only vision commit to duties ranging from touch processing to verbal memory.

Sinha now finds himself in a position to explore how kids' brains adapt to years of blindness and then respond to the onset of sight. He and his coworkers are tracking the progress of 20 children in India, ages 6 to 15, who grew up sightless before the surgical removal of their cataracts. "I'm amazed at how much these kids can do based on vision shortly after cataract surgery," the MIT scientist says. "No one knows if the visual modality will reclaim areas in their brains that it lost to other senses due to blindness"

FACE TIME It's particularly gratifying to observe the success of formerly blind children at recognizing the faces of their family members, physicians, and other familiar people, Sinha says. He estimates that cataract--induced blindness affects as many as 100,000 children in India.

The "fairly crummy" level of visual detail available to most of the Indian children after cataract surgery encourages them to concentrate on the geography of entire faces, while ignoring the nuances of eyes, mouths, noses, or hair, Sinha says. These children also often recognize even partial or faded pictures of familiar faces, indicating that the youngsters refer to a mental catalogue of whole faces, Sinha says.

Babies, whose vision is also blurry, may similarly perceive whole faces rather than specific facial features, he theorizes.

Yet such speculation runs smack into scientists' limited knowledge about the nature of face perception in formerly blind children, as well as in infants (SN: 7/7/01, p. 10).

At least some data on the subject come from studies of Canadians directed by psychologist Daphne Maurer of McGill University in Hamilton, Canada. Children subjected to cataract--induced blindness in only the left eye for the first 2 to 6 months of life lose an element crucial for discerning facial configurations, Maurer's team reports in the October Nature Neuroscience. As teenagers and young adults, these individuals find it difficult to detect differences in the spacing of eyes and other facial features from one person to another.

In contrast, people of the same age who had right-eye cataracts for 2 to 6 months after birth can discern the distance between facial features as well as people with no prior vision problems do.

Despite lacking this face--recognition skill, adults who had left-eye cataracts removed during infancy still manage to recognize their friends and family and don't report any problems in telling familiar faces apart.

Individual facial features evidently guide recognition. McGill psychologist Catherine J. Mondloch and her coworkers found that people deprived of left-eye vision as babies could accurately tell when the researchers had substituted different eyes or mouths on previously seen images of faces or had digitally thinned or fattened the faces.

The results, which so far derive from 10 volunteers born with left-eye cataracts and another 10 born with right-eye cataracts, implicate the brain's right side in expert face processing, Mondloch says. It is only during infancy that visual information entering the left eye goes mainly to the right hemisphere, while the right eye sends most of its visual input to the left hemisphere. Thus, the capacity to notice the spacing of facial features develops only if the right hemisphere receives visual stimulation during that brief period of time. Even then, according to other studies directed by Mondloch, this skill isn't fully developed until about age 18.