Scientists at New York University School of Medicine reveal the important role of early experience in shaping neuronal development and brain plasticity in a new study published in the July 14 issue of the journal Nature.
In mice, the researchers found that sensory deprivation prevented the substantial loss of synapses that typically occurs in growing animals. The effects were most pronounced in the period from young adolescence to adulthood. Synapses are the gaps between neurons through which information travels.
Wen-Biao Gan, Ph.D., Assistant Professor of Physiology and Neuroscience, and his colleagues captured images of brain plasticity--its ability to adapt quickly to ever-changing circumstances--and have started to unravel how this dynamic unfolds. The scientists were able to deliver visible evidence of the effect of sensory deprivation.
It is well known that a growing child learns many skills. "What is less known," says Dr. Gan, "is that during childhood until puberty in the human brain, as well as in the monkey and mouse, you see a substantial loss of neuronal connections." In learning, it appears the brain needs to lose as it gains. He believes this loss may well be the fundamental process underlying the development and plasticity of the brain.
After birth, the number of synapses increases and then decreases sharply. From early childhood to adolescence the synaptic loss could be as much as 50 percent.
Dr. Gan believes that in order for learning to occur, the brain's neurons have to be pruned. "First there is a raw material, and then it is sculpted," he says. In other words, learning isn't only about making new connections between neurons, he says, it also involves carving neuronal connections.
The authors of the new study are Yi Zuo, Guang Yang, Elaine Kwon, and Dr. Gan of the Molecular Neurobiology Program at the Skirball Institute of Biomolecular Medicine at NYU School of Medicine.
To get a glimpse of living neurons in mice, the researchers employed a laborious technique for shaving the skulls of the animals. This creates an ultrathin window on the brain through which one can peer using a sophisticated optical technique called two-photon fluorescence microscopy. Dr. Gan looked at dendritic spines, which are thorny nubs found all along the branches of neurons. Spines, which are continuously formed and eliminated, are where synapses are made.
Since mice use their whiskers to explore their world, Dr. Gan altered their experience by trimming the whiskers for two weeks on one side of the mice's snouts. The spines of these mice were then compared to spines in mice of the same age with untrimmed whiskers. Young mice who kept their whiskers showed more spine loss than their whisker-trimmed litter-mates.
In the adult age group, whisker trimming for two weeks appeared to have no significant effect on spine loss. When the sensory deprivation continued for two months, however, spine loss was slightly reduced in adult animals as well. The scientists therefore found that the period of young adolescence to adulthood was particularly susceptible to sensory deprivation.
Interestingly, in adolescent mice the effects of sensory deprivation on spine loss could be largely reversed if whiskers were allowed to re-grow during a subsequent recovery period, says Dr. Gan. However, the effects of sensory deprivation in young adolescence couldn't be reversed if sensory recovery occurred after the mice reached adulthood. These findings suggest "that childhood experience has a long lasting and perhaps permanent impact on later life," he says.