How the brain makes memories: rhythmically!
The brain learns through changes in the strength of its synapses — the connections between neurons — in response to stimuli.
Now, in a discovery that challenges conventional wisdom on the brain mechanisms of learning, UCLA neuro-physicists 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, which provide a grand-unified theory of the mechanisms that underlie learning in the brain, may lead to possible new therapies for treating learning disabilities.
The study appears in the current issue of the journal Frontiers in Computational Neuroscience.
“Many people have learning and memory disorders, and beyond that group, most of us are not Einstein or Mozart,” said Mayank R. Mehta, the paper’s senior author and an associate professor in UCLA’s 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 — known as synaptic plasticity — 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 (e.g., 100 spikes per second) strengthened the connecting synapse, while low-frequency stimulation (e.g., one spike per second) reduced synaptic strength.
These earlier experiments used hundreds of consecutive spikes in the very high-frequency range to induce plasticity. Yet when the brain is activated 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, said 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.”
Until now, researchers had been unable to conduct experiments that simulated more naturally occurring levels. But Mehta and co-author Arvind Kumar, a former postdoctoral fellow in his lab, were able to obtain these measurements for the first time using a sophisticated mathematical model they developed and validated with experimental data.
Contrary to what was previously assumed, Mehta and Kumar found that when it comes to stimulating synapses with naturally occurring spike patterns, stimulating the neurons at the highest frequencies was not the best way to increase synaptic strength.