Stimulation: Too much could hurt when you are young.

In an experiment that is sure to make friends in the music, fitness and whisker-stimulation industries, Nature reports that steady stimulation blocks growth of blood vessels in the brains of infant mice. You’ve probably heard that exercise is good for mental health and growing new neurons. But when young mice jogged on an exercise wheel for three hours a day, blood vessels stopped forming in the motor cortex, the hunk of brain closely tied with movement. The same thing happened in a sensory section of the brain when the mouse’s whiskers — a key part of the animal’s sensory apparatus — were activated by a breeze. Before you jump to conclusions, remember that these changes followed hours of persistent stimulus. “Normal activity, or a very mild boost of activity, does not have any impact on the formation of new blood vessels,” senior author Jaime Grutzendler, associate professor of neurology and of neurobiology at Yale University, told us. “You have to have persistent stimuli for it to become pathological and to see inhibition of blood vessel formation.” Overall, the slowdown in vessel growth “represents a 70 percent reduction in the numbers of new vascular branches formed and an 80 percent reduction in length growth when compared to unmanipulated animals,” the scientists reported. The authors wrote that blood vessels are particularly important to the brain, which, “unlike other organs, has high energetic demand, minimal energy storage,” and no reserve of tiny vessels to pitch in when needed.

Could jogging be fogging?

In adults, new blood vessels are only wanted after injury, but the story is different around the time of birth, when the forming brain must install plumbing to deliver blood — and so capillaries are on a growth spurt during the first month or so of mouse-life. In people, one month corresponds to about two years, but it’s possible that our brain blood vessels grow for the first five or six years, Grutzendler says, “so exposure later than age two could also have this effect.” For a mouse, the first month of life is a “critical period for blood vessel development,” Grutzendler says. When the over-stimulation lasted five days, blood vessel formation returned to normal within a month because the blood vessels could still grow during the critical period. But if the stimulation lasted 15 days, the reduction was still visible five months later. “If you disrupt the normal formation of blood vessels at that specific time point and it extends through the end of the critical period, you go through life with fewer blood vessels,” Grutzendler says. “That’s something people have not known about before.”

When sound becomes noise

The most reliable reduction in blood vessel formation came from the 10-hour barrage of sound — mainly moderately loud “white noise,” and natural sounds, including some mouse-talk. The blockage of blood vessels “wasn’t related to volume, it was related to persistence,” Grutzendler says. And the blockage, in each case, was found in the section of brain linked to the sensation or activity that was overstimulated. We mentioned that some parents use white noise generators: to mask noise so their babies can sleep, and Grutzendler thought this “could be a problem, along with apps, all kind of things. Visual over-stimulation — TV or video games — seems to be very common at a certain age. Certainly people play music continuously next to babies; it must be very common throughout the world.” The Nature study did not look at behavior, but the mice “were not obviously impaired in any way,” Grutzendler says. However, a shortage of blood vessels could harm the brain during physical activity. “I’m not saying it leads to cognitive impairment, but it’s intriguing. This could be associated with other pathologies of aging, when you normally start to lose blood vessels in the brain.” Grutzendler adds that scientists have previously found that mice were less able to differentiate sounds after repetitive noise. “We may have provided the answer to their question.” Mice are not people, and “I prefer not to make people paranoid,” Grutzendler concludes, “but it’s clearly important to make people aware that persistent, repetitive stimuli could be bad in subtle ways.”