In a first of its kind study, researchers at Massachusetts Institute of Technology have found a way to induce gamma waves by shining laser light directly onto the brains of mice, which could lead to new treatments of brain-related disorders.
For a long time, scientists have studied high-frequency brain waves, known as gamma oscillations, believing them to be crucial to consciousness, attention, learning and memory.
But, in the new study, researchers used a newly developed technology known as optogenetics, which combines genetic engineering with light to manipulate the activity of individual nerve cells.
The study could explain how the brain produces gamma waves and provides new evidence of the role they play in regulating brain functions, which could someday lead to new treatments for a range of brain-related disorders.
"Gamma waves are known to be [disrupted] in people with schizophrenia and other psychiatric and neurological diseases. This new tool will give us a great chance to probe the function of these circuits," Nature quoted Li-Huei Tsai, Picower Professor of Neuroscience and a Howard Hughes Medical Institute investigator, as saying.
Gamma oscillations reflect the synchronous activity of large interconnected networks of neurons, firing together at frequencies ranging from 20 to 80 cycles per second.
"These oscillations are thought to be controlled by a specific class of inhibitory cells known as fast-spiking interneurons. But until now, a direct test of this idea was not possible," says Jessica Cardin, co-lead author on the study.
For finding out which neurons drive the oscillations, the researchers used a protein called channelrhodopsin-2 (ChR2), which can sensitise neurons to light.
"By combining several genetic tricks, we were able to express ChR2 in different classes of neurons, allowing us to manipulate their activity with precise timing via a laser and an optical fibre over the brain," explains co-lead author Marie Carlen.
The trick for inducing gamma waves was the selective activation of the "fast-spiking" interneurons, named for their characteristic pattern of electrical activity.
When these cells were driven with high frequency laser pulses, the illuminated region of cortex started to produce gamma oscillations.
"We've shown for the first time that it is possible to induce a specific brain state by activating a specific cell type" said co-author Christopher Moore.
On the other hand, no gamma oscillations were induced when the fast-spiking interneurons were activated at low frequencies, or when a different class of neurons was activated.
Also, the researchers showed that these brain rhythms regulate the processing of sensory signals.
They found that the brain's response to a tactile stimulus was greater or smaller depending on exactly where the stimulus occurred within the oscillation cycle.
"It supports the idea that these synchronous oscillations are important for controlling how we perceive stimuli. Gamma rhythms might serve to make a sound louder, or a visual input brighter, all based on how these patterns regulate brain circuits," said Moore.
The study was published in the latest online issue of Nature.