CHICAGO – One-fifth of infants and toddlers under age 2 have a television in their bedrooms, according to a new study that suggests saturation with media could affect brain development as more shows start to target America’s youngest children.

The telephone survey, among the first in-depth looks at the viewing habits of very young children, indicates that many parents have ignored or are unaware of pediatric guidelines recommending limits on kids’ TV time.

In addition to the youngest group, 43 percent of 3- to 4-year-olds have TVs in their rooms, potentially setting up an unhealthy habit, the authors wrote. Previous studies have found that bedroom TVs are linked with childhood obesity, inactivity and low scores on reading and math tests. Research also suggests that extensive viewing before age 3 may cause attention problems later on.

The children’s viewing practices reflected their parents’ often guilty reliance on TV, said authors of the study, to be published Monday in the journal Pediatrics. Asked why they put TVs in their children’s bedrooms, most parents said it was so they and other family members could watch their own shows.

“A lot of these parents are kind of checking out,” said study co-author Elizabeth Vandewater, a professor of human development and family sciences at the University of Texas at Austin. “It’s not really neglect, it’s more like absentee parenting.”

Although the American Academy of Pediatrics discourages any TV viewing during the first two years of life, some researchers believe that’s an unrealistic goal. Nearly 70 percent of children in that age group watch some television, according to the new study.

Widespread marketing claims about the value of programming for toddlers and infants may play a role, experts said.

The number of shows aimed at babies has exploded in the last decade, with video titles such as “Brainy Baby” and “Baby Einstein” joined last year by BabyFirstTV, billed as the first satellite channel catering to infants. Makers of the shows often bill them as educational aids, though researchers say there is no solid evidence to back such claims.

The landscape has shifted dramatically since 1961, when experts say most children did not watch TV until they were nearly 3 years old. Now, studies suggest that children start watching TV or videos by the time they’re 9 months old, on average.

“There’s a real historical change going on, and we’re in the middle of it,” said Daniel Anderson, a professor of psychology who specializes in children and television at the University of Massachusetts in Amherst.

The news for parents isn’t all bad. The new study found that children ages 3 to 6 watched TV for less than the recommended limit of two hours a day – though some experts doubted that figure because it’s based solely on parents’ reports.

Not all viewing time is equal, researchers said. Parents who watch TV with their children or toddlers have more chance to talk about what the kids are seeing, although studies have shown that parents respond to kids less when the TV is on.

Trice Canady said all of her children (ages 4 through 13) have TVs in their bedrooms, but most of their viewing takes place in the family room.

“It’s my favorite time of day when we’re all gathered in front of the TV,” Canady said.

That doesn’t mean that other parents share her point of view, she conceded.

“I get a tremendous amount of grief,” she said. “They say, “I can’t believe you let them each have a TV.’ Or they’ll say that they are spoiled. But all my kids are honor roll students. They know that school comes first.”

Canady estimated that her children watch about two hours a day, and they strictly adhere to “lights out” at 9:30 p.m.

What’s OK for preschool kids may not be good for infants, however. Television’s potentially negative effects change as a child ages, researchers believe.

Before the age of 30 months or so, most children absorb little of what happens on the screen. They may even lack the basic visual ability to decode a TV’s oscillating lights into meaningful images. That means the time babies spend with television is probably wasted, leaving less time to spend on activities that are crucial to development.

“If they’re spending an hour in front of the TV, that’s an hour that they’re not playing with blocks or playing with Mom,” said Dr. Michael Rich, director of the Center on Media and Child health at Children’s Hospital in Boston.

Children older than 2 can comprehend and learn from TV, which makes the specific content of programs more important as a child ages. Some research suggests that children can pick up reading or math skills from shows such as “Sesame Street” and learn more about helping others by watching Barney the dinosaur.

In contrast, brain-scan studies have found that older children exposed to violent programs show activity in a part of the brain that’s active in rape victims and other sufferers of post-traumatic stress disorder.

“That begins to explain other studies that show kids who watch violence are more likely to behave aggressively,” said John P. Murray, a professor of developmental psychology at Kansas State University who led the brain-scan research.

Makers of baby videos dispute the notion of a cut-off age below which infants can’t learn from TV. Dennis Fedoruk, president and chief executive officer of the Georgia-based Brainy Baby Co., said his company encourages parents to use its products “as they would any other learning tool, in moderation.”

Asked if he had scientific backing for the claim that the videos educate infants, Fedoruk said, “There’s not any scientific evidence, but I would say, go ask the tens of thousands of moms who in their own minds and hearts know that their children can learn from our products.

“I think Mom knows best, is what it comes down to.”

Last year, the Campaign for a Commercial-Free Childhood filed a complaint with the Federal Trade Commission against Brainy Baby, Baby Einstein Co. and BabyFirstTV, saying their claims of educational benefits for infants amounted to false and deceptive advertising.

Yet some experts point out that the American Academy of Pediatrics guidelines for TV viewing are not based on foolproof evidence either. That’s especially true of the group’s suggestion that infants watch zero TV, said study co-author Victoria Rideout, a vice president with the Kaiser Family Foundation.

“I think they made those recommendations out of an abundance of caution, but not because they had a slam-dunk case,” Rideout said.

That raises a dilemma for scientists such as Vandewater of Texas, who said she had a hard time deciding whether the pediatricians’ guidelines have merit. Then she thought of a useful metaphor for early TV viewing.

“Imagine there was a river right in front of your house and someone said if you dip your baby in the river, it’s possible you could enhance their development later, but it’s also possible you could harm them,” Vandewater said. “What would you choose?

“I think I’d say, “You know what? I’ll just take normal development, thank you very much.”‘