The idea that the adult brain changes with experience was once a radical idea, but it is now well accepted that certain areas—say, the motor cortex, when learning a new physical skill—can grow new neurons or create stronger connections.
Now scientists report that the brain is even more mutable than suspected. Thanks to an unconventional research technique, neuroscientists have found the first physical proof that new experiences and information have wide-ranging effects throughout both hemispheres of the brain, rather than just creating connections in one discrete area.
The story begins in the hippocampus, the area of the brain associated with short-term memory. In the past, researchers have electrically stimulated slices of disembodied hippocampus and seen how stimulation changes the structure of nearby neurons. But the new study took a different tack. Led by Santiago Canals, a biological cyberneticist currently at the Institute of Neurosciences in Alicante, Spain, the team set aside the dissected hippocampi in favor of a more true-to-life approach. After implanting electrodes in live rats, the group used a combination of functional MRI, electroencephalography (EEG) and microstimulation—triggering nerve cells with small doses of electric current—to trace in real time what happened to neuronal structures in the rats’ brains when neurons in the hippocampus were stimulated. In contrast to studying the slices, this method allowed the scientists to see what happened in the hippocampus in context with what was going on all over the brain—like comparing a 2-D drawing of a bedroom with a 3-D rendering of the whole house.
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“We have learned that what we call neuronal plasticity isn’t exclusive to individual synapses or even the neurons where they contact but rather occurs throughout the functional network in which synapses and neurons are embedded,” Canals says. “Those networks are absent in brain slices, so they couldn’t be studied before.”
By showing how activity in the hippocampus causes widespread changes in brain structure, Canals says the findings could explain why new memories are at first dependent on the hippocampus but can eventually be recalled without triggering that part of the brain at all.