A baby views patterns that appear to change once binocular vision has developed.… (Gabor Jando, University…)
Does depth perception develop in humans as a result of nature or nurture?
It's a question scientists have wondered about. And a new study comes to a surprising conclusion: Babies acquire binocular vision as a result of viewing the world around them, not merely thanks to genetic programming.
"My guess was that it was going to be something in between nature and nurture," said study leader Ilona Kovacs, a psychologist at the Budapest University of Technology and Economics in Hungary. Instead, it appeared that nurture — the environmental stimulation a baby received — was the key.
The finding, reported this week in Proceedings of the National Academy of Sciences, extends scientists' knowledge about just how much the visual system in the brain relies on what we see and when we see it. It may also lead to new tests for visual dysfunction early in life.
Vision is a widely used model for studying brain development. One of its advantages is that, unlike sound, visual input does not reach an infant in the womb. That allows scientists to be sure that a newborn's visual system is unadulterated.
Binocularity is a particularly attractive characteristic of the visual system to study because, in normal babies, it comes on suddenly a dependable three to four months after birth. This gives scientists a clear milestone to observe.
A growing body of research has shown that early life experiences play a crucial role in the development of normal brain function. In the visual system in particular, a "critical period" takes place during infancy in which the part of the brain that processes what we see becomes highly plastic. During that time, infants are particularly vulnerable to developing visual deficits, like "lazy eye," if their vision is disrupted by congenital cataracts or an eye injury. If problems are not fixed during the critical period, they often remain for life.
Because early life is so important for brain development, studying babies is key. The trick for scientists, however, is coming up with a way to test their ideas. Normal infants all develop on about the same timeline, making it difficult to compare two different groups.
Kovacs and her colleagues got around this problem by comparing the brain activity of two groups of healthy babies: Some were born prematurely and others were born at full term. This allowed the scientists to explore whether development of binocular vision depended on the amount of time the babies were out of the womb, or on the amount of time they had been developing since conception. The nature side of the equation was presumed to be about the same for both groups.
During the experiment, 30 newborn babies viewed a screen with a pattern of dots that changed in a way that only people with binocular vision could detect. To tell whether they noticed the change, the scientists measured the babies' brain activity with electrodes that were connected to their heads. The babies were tested about once a month until their binocular vision kicked in.
The study design may have been clever, but it was also difficult to carry out. Finding healthy preterm infants is a tough proposition, so the study took more than six years.
But when the data were in, the results were clear: Postnatal age determined when binocular vision arrived, with both groups of babies developing it about four months after birth.
And when the researchers showed the babies a flashing checkerboard — a widely used test of basic vision function — the responses of the two groups diverged. That suggests the role of visual experience is specific to complex visual functions, like binocular vision.
Kovacs' results may have clinical applications as well. She believes her test could be used as a screening tool for early life vision disturbances that cause a failure of depth perception, like severe amblyopia, raising the possibility that such problems could be fixed if detected early enough.
Takao Hensch, an expert in the biology of early brain development at Harvard Medical School who was not involved in the study, said the research provides a rare glimpse into how early experiences can change our brains for life.
"It's an unusual opportunity to observe in humans what might happen during these critical developmental milestones," he said. "And since you know the first photons are hitting the visual system at birth, it's a very clever, very clean experiment where you can look at the effects of experience starting from nothing."