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Movies of Intact Brain Reveal How Sensory Experience Shapes Neural Connections

Cold Spring Harbor, NY - Rats, mice, and other rodent denizens of the shadows use their whiskers to explore their surroundings by feel. Now, researchers at Cold Spring Harbor Laboratory have captured time-lapse images that show how neurons in a young rat's brain change as the animal experiences its world through its whiskers for the first time.

The study, published in tomorrow's issue of Nature, demonstrates that sensory experience during a "critical period" soon after birth influences brain architecture by changing the connections among neurons. A similar kind of experience-dependent "synaptic plasticity" undoubtedly shapes the formation of neural networks that underlie not only touch perception in rodents but also vision, audition, olfaction, learning, and memory in many animals, including humans.

"The sense of touch that rodents get from their whiskers is so acute that these animals can essentially read our fingerprints," says Karel Svoboda, the leader of the new study. "We use the rodent whisker system as a model for understanding in more general terms how sensory experience is required for the brain to wire up correctly."

Svoboda and his colleagues were able to peer under the surface of a rat's brain and capture images of neurons in the act of reshaping a neural network by using a custom-built optical imaging microscope (a "two-photon excitation laser scanning" microscope). Time-lapse imaging of fluorescently labeled neurons showed that dendritic protrusions-which form connections between neurons called synapses-rapidly appeared, disappeared, or changed shape. Depriving an animal of sensory experience by trimming its whiskers greatly decreased the motility of dendritic protrusions and dramatically perturbed the function of the corresponding brain region that processes sensory information from whiskers.

Surprisingly, sensory deprivation did not alter the number, length, or shape of dendritic protrusions. This finding, combined with the observed decrease in the motility of dendritic protrusions in sensory-deprived animals, led the scientists to conclude that in normal animals, synaptic connections are relatively dynamic during the critical period while in sensory-deprived animals, synaptic connections are more static during this period.

Svoboda and his colleagues suggest that sensory experience stimulates the rapid formation and pruning of synaptic connections and that this reshaping process is required for the construction of fully functional neural networks in the cortex, the seat of most "higher" brain function such as perception and cognition.

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