Living in poverty is a major factor in the development of "toxic stress," which can cause irreversible, negative effects in the brains of children through 5 years old, according to Valley physicians.

Children exposed to violence, substance abuse, mental illness, physical and sexual abuse, or who have neglectful or absent parents, are more likely to develop attention-deficit hyperactivity disorder or depression, have lower IQs and are more at risk for chronic adult conditions such as diabetes, heart disease and alcoholism.

Many of the children who visit Dr. Jessica Pagana DeFazio's medical practice in Sunbury are from families living in poverty.

"It's rampant," DeFazio said of toxic stress in children. "This is what extreme poverty does to people."

More than 10 percent of Valley families live in poverty, which for a family of three is an $18,310 household income, and a family of four, $22,050, according to the U.S. Department of Health and Human Services.

Union has the highest poverty rate among the four Valley counties, at 13.2 percent, according to a 2007 report by the U.S. Department of Agriculture's Economic Research Service.

Montour has the lowest poverty percentage, 10.4, of the Valley counties.

The effects of toxic stress, DeFazio said, are permanent.

"It leaves an imprint on the brain that you cannot take off."

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On its way to reaching a trillion brain cells in adulthood, the brain forms 700 new neural connections every second during the first few years of life.

"The first three years of life is when many (neural connections) are formed," said Dr. Heather Hoover, a pediatric neuropsychologist at Geisinger Medical Center's Janet Weis Children's Hospital in Danville.

Synapses, which allow for functional communication between brain cells, form naturally, Hoover said.

But environmental stimulation will determine whether the synapses will be strong or weak.

Brain development occurs in three stages, beginning at three or four weeks after conception, with the development of vision and hearing. Language development is then followed by cognitive function, which is the problem-solving stage in which children learn cause and effect.