Pesticide exposure linked to brain activity differences in adolescents

A group of California teenagers exposed to common agricultural pesticides before birth had distinctive reductions in certain types of brain activity, according to a new study published in the Proceedings of the National Academy of Sciences.

The study, which is the first to link differences in adolescents’ brain activity to their level of prenatal pesticide exposure, adds to a large body of research about the health effects of organophosphatepesticides. Organophosphates are a group of insecticides sprayed on many fruit and vegetable crops. Because of toxicity concerns, the federal Environmental Protection Agency banned indoor and residential uses of these pesticides in 2000, and in May, the California EPA prohibited all uses of one organophosphate compound, chlorpyrifos.

But many of these insecticides are still used in farming, and most people are exposed to them through their diets. People who live in farming communities also inhale the pesticides when they blow off nearby fields.

“There have been several hypotheses about what effect an insult like organophosphate exposure would have on the brain,” said experimental psychologist Joseph Baker, PhD, a co-author of the new study. “For instance, is it less overall activity, or does the brain respond by having more but more-diffuse activity? The answers to these basic questions were unknown.”

The study found that during executive-function tasks, brain activation was decreased in the prefrontal cortex in teens with higher levels of prenatal pesticide exposure, as compared to teens whose exposure was lower. “Executive function” refers to one’s ability to prioritize tasks in the short run in order to reach long-term goals, such as planning and carrying out the steps to complete a school assignment. The prefrontal cortex is a region at the very front of the brain that is responsible for much of our conscious thought and decision making.

The research was a collaboration between scientists at Stanford and the University of California, Berkeley. The Berkeley team has been collecting data since 1999 on pesticides’ effects on mothers and children in California’s Salinas Valley, a major producer of crops such as lettuce, strawberries, broccoli and spinach. The researchers used data on mothers’ place of residence during pregnancy to estimate the children’s pesticide exposures in utero; some study participants were estimated to have relatively low pesticide exposure levels while for others, exposure was much higher.

The Stanford scientists joined the work because of their expertise in a brain-scanning technique called functional near-infrared spectroscopy. They scanned 95 teens whose mothers lived in the Salinas Valley during pregnancy and who had continued to reside in Salinas throughout their lives. The scans were done while the teens performed several kinds of cognitive tasks.

The team saw brain-activity differences during executive-function tasks that correlated to the teens’ levels of prenatal pesticide exposure. The research included tests designed to measure cognitive flexibility and working memory, which is the ability to hold and manipulate items in the brain for short periods of time.

“It’s really quite amazing that we see a long-term association between these exposures and brain function,” said neuroscientist Allan Reiss, MD, a co-author of the study.

Reiss and Baker hope their findings will provide a foundation for future research to understand how the pesticides change the activity of the human brain. They also hope the findings will spark more conversation about the trade-offs of using the pesticides.

“These findings hit at the intersection of brain development and function, the socioeconomic status of our farm labor force, and the economics of crop production,” Reiss said. “In the end, we have to ask what’s more expensive: to have a greater push toward organic farming or to have more individuals who need assistance because they are dealing with long-term effects of exposure to pesticides. The latter might be harder to measure than seasonal profits, but it potentially represents a larger cost to the individual and our communities in the end.”

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