Too Much too Young
(appeared on 11th Dec 2013)

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Excessive stimulation of young minds may not be a good thing, says S.Ananthanarayanan.

Current DIY child psychology has it that ‘catch them young’ not only gives children a ‘competitive advantage’ but also helps their brain grow at the only time in their lives that this is possible. Commercial minds have latched on and new parents wonder if they are failing the child if she is not in the most advertised nursery by the time she can walk. But the parental anxiety to give their child the best in life often starts earlier, with constant attention and aural, visual and tactile stimulation right from birth.

Christina Whiteus, Catarina Freitas and Jaime Grutzendler at the Yale School of Medicine, Connecticut, USA, report in the journal, Nature, their finding that “Chronic over-stimulation early in life can permanently impair the development of tiny blood vessels in the brain.” Conversely, neither reduction of sensory input nor moderately increased activity by environmental enrichment affects the same development. The context was the observation that activity dependant development of nerve cell patterns during infancy coincides with the development of finer blood vessels in the brain. This had raised a question of whether activity of brain cells could also affect the growth of the blood vessel system.

The authors note that given the brain’s high energy consumption and susceptibility to seizure or tissue death by lack of blood supply, development of an adequate network of the narrowest blood vessels to match metabolic demands is vital. Although the major blood vessels in the brain are formed even before birth, the final development of the blood supply system continues through early infancy, and is concurrent with development of the nerve cell connections of the brain. As there are common regulatory mechanisms that mediate growth of blood vessels and nerve tissue, the question of whether activity of the brain nerve cells affects the development of the blood vessel network arises and has not been investigated.

To address the question, the authors investigated the effect of physical stimulation and an enriched environment, which promotes neural activity, on the development of the brain blood vessel system in experimental mice. The first trial was of the effect of reduction of sensory inputs to the brain, by trimming both the whiskers of newborn mice, on the fifteenth day after birth. Tests showed that this had no effect on the development of blood vessels in the part of the brain involved. While trimming the whiskers had the effect of reduction of stimulation, even moderate stimulation, by providing the young mice with an ‘enriched’ environment, which is to provide them with tasks and puzzles, or even other mice to associate with, also did not affect the rate of blood vessel growth. This trial thus showed that ordinary sensory and neural activity did not have an effect on brain blood vessel development.

Further trials, however, produced unexpected results. More persistent and repeated activity as also exposure to repetitive sounds and noise over an extended period (ten hours) led to positive reduction in the development of blood This effect became greater when the stimulation by activity and sounds was extended. And what is more, the effect was specific to the part of the brain that was engaged by the enhanced activity. When whiskers of only one side were trimmed and the remaining whiskers stimulated by an air current, for ten hours a day for eight days, there was reduction in blood vessel development in the part of the brain associated with the stimulated whiskers. Three hours on the treadmill every day for five days resulted in reducing blood vessel growth in the area associated with motor functions.

Over the ten day period from the fifteenth to the twenty fifth day after birth, repetitive stimulation led to a reduction of 13% in the branching and 8% in the in length of blood vessel development in the stimulated mice, as compared to the mice that were left alone. This difference amounts to a 70% reduction in the new branches and an 80% reduction in the increase in length in blood vessels, as a result of the persistent stimulation. The slowdown of blood vessel development was seen to persist and did not recover even after extended over-stimulation was stopped, which shows that the effect is long lasting. In contrast, continuous stimulation of older mice did not have these effects, which seems to be natural, as branching and growth of blood vessels in the brain is not much after early infancy.

Next, the team tested the effect of epileptic seizures on the development of blood vessels. It was found that seizures induced by the drug, pilocarpine, which stimulates brain cells, or by the tetanus toxin, which acts by blocking inhibiting action, led to reduction of blood vessel density in infant mice. But injection of another drug, which blocks transmission of excitation in nerves, had no effect on blood vessels. This supports the view that it is hyperactivity that affects the development of the fine blood vessel system in brains of newborns.

Further tests also showed that the effect on growth of blood vessels was a result of the Nitric Oxide that is released during neural activity. While moderate levels of Nitric Oxide support blood vessel formation, increased levels, due to hyperactivity, start blocking blood vessel development. In fact, it was clear that Nitric Oxide was the relevant factor because injecting an agent that suppressed the enzyme that helps in production of Nitric Acid completely blocked the reduction in blood vessel formation that was caused by hyperactivity.

While the studies have been on new born mice, the relevance to the development of human brain is obvious. That very early learning, before the onset of the correct stage in development is of little avail is well known. For instance it is known that a child is ready to learn to read at age six. A child who is taught earlier can learn, no doubt, but may have no advantage over another child who was taught only when she was at the appropriate age. On the other hand, the study now reported says sensory and auditory stimulation of young children may be positively and permanently harmful, in a physiological sense, apart from psychological harm.

“These findings raise the concern that early childhood seizures or exposure to repetitive auditory and other sensory-motor stimuli, which are common in modern society could have lifelong repercussions,” the authors say in the paper.

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