Prolonged exposure to repetitive sounds can reduce the formation of blood vessels in the brains of newborn mice, Yale researchers have found, causing changes that may ultimately make them more vulnerable to stresses and aging.
The study, published Wednesday in the journal Nature, raises the provocative possibility that auditory experiences frequently encountered in modern life, such as blaring music or white noise machines, could have a long-lasting impact on the brains of babies, too.
“It’s definitely a cautionary observation, that these stimulations that we have in modern society should not be abused,” said Dr. Jaime Grutzendler, associate professor of neurology and neurobiology at the Yale University School of Medicine. “I would say the auditory stimulation we did in mice is well within the exposures that humans are exposed to in modern society—and even less. ... It’s not the volume; it’s the persistence.”
Grutzendler said the findings should not send parents into a panic; these are studies on mice, not humans. But the result raises important questions about early human brain development that he hopes will be followed up by other researchers.
When he and his colleagues first designed the experiments, they were simply continuing their studies of the growth of blood vessels in the brain. They had recently discovered that although much of the brain’s network of branching capillaries and vessels were in place before birth, some growth continues for the first month of a mouse’s life—analogous to about the first two years of life in a baby.
They expected that stimulating the newborn mice with activity might increase the blood vessel growth to an area, because those brain areas would become more active. To their surprise, the opposite occurred. They overstimulated the mice three different ways: by having them run on a treadmill for three hours a day, subjecting them to 10 hours of tones, sounds, and white noise each day, or overstimulating their whiskers. All three interventions led to reduced blood vessel growth in the areas of the brain involved in the activity. The researchers also triggered seizures in the mice, which similarly reduced blood vessel growth.
They found this reduction in blood vessel growth was reversible-- to a point. The brain’s blood vessel architecture returned to normal in newborn mice that received only five days of repetitive auditory noises. But those that received 15 days of it had permanent reductions in the network of blood vessels in their brains.
The researchers did not do behavioral studies, so they did not measure whether the reduction led to cognitive changes. But Grutzendler said mice with a sparser network of blood vessels in their brain are more vulnerable to stresses. Simply subjecting mice with reduced blood vessels to the stressful conditions found at high altitudes resulted in them losing some of the brain cell connections in their brain.
Now, Grutzendler hopes to better understand the effect and search for a threshold where stimulation switches from promoting brain development to impeding it—years of research have shown that stimulating the brain during a “critical period” when the young brain is most plastic and adaptable is essential for proper brain development.
He pointed out that his team also hadn’t yet examined the way in which people may be most overstimulated as children—as they are entranced by television, iPads, or the many screens that fill our lives.