Attempts to control the brain through direct stimulation have a long history in neuroscientific study, dating back to the ancient Egyptians, said Christopher Moore, associate professor of neuroscience. Moore and his colleagues have continued this line of inquiry in a study, published in Nature Neuroscience last month, in which researchers enhanced touch sensitivity in mice by triggering a specific brain rhythm.
Using optogenetics — a method of neuron control developed and popularized in the early 2000s — researchers sensitized neurons in the mice’s cortices to a specific light. When researchers shined a blue light on them, these cortex neurons fired.
“The main advantage to using optogenetics is that you can have very precise temporal control over a population of neurons,” said Joshua Siegle ’07, co-lead author of the study, who is currently a graduate student at MIT.
By stimulating the neurons with certain patterns, the neuroscientists changed the synchrony and frequency of neuronal firing in the mice and produced gamma waves — brain waves that are thought to be vital for attentional processes.
Gamma waves run at the highest frequency of any brain rhythm — about 40 hertz in mice, the researchers said. The role of gamma waves in brain function has been a long-debated matter in neuroscience, said Moore, the study’s senior author.
Some researchers believe the synchrony of these waves is the basis for all brain functioning, while others argue the speed at which neurons fire is most important, he added.
The researchers aimed to test this idea causally by recording mouse whisker vibrations in a naturalistic setting. They played these vibrations back to the mice when gamma rhythms both were and were not present. When gamma rhythms were present, the mice were able to detect fainter vibrations than when the rhythms were absent.
“Our data very strongly argue that when we created a situation that did not increase firing rate on average but did increase synchrony, we got better perception,” Moore said.
In the experiment’s second part, the researchers shifted the timing between the emergence of gamma oscillations and tactile stimulus presentation to determine the time frame in which sensory sensitivity was most increased. The researchers found that mouse sensitivity was increased only when gamma waves were elicited within 25 milliseconds of stimulus presentation. “If a signal arrives in this window of opportunity, it becomes easier to detect,” Siegle said, adding that mice presented with tactile stimuli outside of this window did not demonstrate an increase in sensitivity.
This study provides a rigorous and convincing report of the role gamma waves play in the brain, said Hava Siegelmann, director of the Biologically Inspired Neural Dynamical Systems lab at the University of Massachusetts at Amherst, who was not involved in the study. “I think this is something that has been making us neuroscientists wonder for a long time. … These methods are beautiful,” she added.
The team plans to continue studying this form of attention and perception in mice. “The next big step … is to look and see how multiple areas interact on the timescale of gamma,” Siegle said. While this study focused on only one area of the brain, Siegle added, future work should be dedicated to observing the interaction of gamma oscillations in multiple regions of the brain.
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