(Great Neck, N.Y. - May 13, 2009) — For half a century, scientists have believed that high-frequency brain waves, known as gamma oscillations, were crucial to consciousness, attention, learning and memory. Gamma waves are known to be disrupted in people with schizophrenia and other psychiatric and neurological diseases.
Now, for the first time, researchers at the Massachusetts Institute of Technology have found a way to induce these waves by shining laser light directly onto the brains of mice.
NARSAD Young Investigator Eva Marie Carlen, Ph.D., of MIT’s Picower Institute for Learning and Memory, is co-lead author of a paper about the finding, which appears in the April 26 online issue of the journal Nature.
Dr. Carlen collaborated with researchers from MIT, Stanford University and the University of Pennsylvania to take advantage of a newly developed technology known as optogenetics, which combines genetic engineering with light to manipulate the activity of individual nerve cells. It helps to explain how the brain produces gamma waves and provides new evidence of the role they play in regulating brain functions - insights that could someday lead to new treatments for a range of brain-related disorders.
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.
To determine which neurons are responsible for driving the oscillations, the researchers used a protein called channelrhodopsin-2 (ChR2), which can sensitize neurons to light. By combining several genetic tricks, they were able to induce expression of ChR2 in different classes of neurons, allowing them to manipulate activity with precise timing via a laser and an optical fiber over the brain. 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.
The researchers showed for the first time that it is possible to induce a specific brain state by activating a specific cell type. In contrast, no gamma oscillations were induced when the fast-spiking interneurons were activated at low frequencies, or when a different class of neurons was activated. The authors further 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, which 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.