THE BRAINS LEARNS THROUGH CHANGES IN THE STRENGTH OF ITS SYNAPESES in response to stimuli. Now, in a discovery that challenges conventional wisdom on the brain mechanisms of learning, UCLA neurophysicists have found there is an optimal brain "rhythm," or frequency, for changing synaptic strength. And further, like stations on a radio dial, each synapse is tuned to a different optimal frequency for learning.
The findings, published in the journal Frontiers in Computational Neuroscience, may lead to possible new therapies for treating learning disabilities.
"Many people have learning and memory disorders," says Mayank R. Mehta, Ph.D., associate professor in the departments of neurology, neurobiology, physics and astronomy. "Our work suggests that some problems with learning and memory are caused by synapses not being tuned to the right frequency."
A change in the strength of a synapse in response to stimuli is induced through so-called "spike trains," series of neural signals that occur with varying frequency and timing. Previous experiments demonstrated that stimulating neurons at a very high frequency strengthened the connecting synapse, while low-frequency stimulation reduced synaptic strength.
During real-life behavioral tasks, neurons fire only about 10 consecutive spikes, not several hundred. And they do so at a much lower frequency — typically in the 50-spikes-per-second range. In other words, says Dr. Mehta, "spike frequency refers to how fast the spikes come. Ten spikes could be delivered at a frequency of 100 spikes a second or at a frequency of one spike per second."
"The expectation, based on previous studies, was that if you drove the synapse at a higher frequency, the effect on synaptic strengthening, or learning, would be at least as good as, if not better than, the naturally occurring lower frequency," Dr. Mehta says. "To our surprise, we found that beyond the optimal frequency, synaptic strengthening actually declined as the frequencies got higher."
Not only does each synapse have a preferred frequency for optimal learning, but also for the best effect, the frequency needs to be perfectly rhythmic — timed at exact intervals. Even at the optimal frequency, if the rhythm was thrown off, synaptic learning was substantially diminished.
Their research also showed that once a synapse learns, its optimal frequency changes. This learning-induced "detuning" has important implications for treating disorders related to forgetting, such as post-traumatic stress disorder, the researchers said.