When individual neurons fire independently, their electrical recordings sound like radio static, all noise and no signal. When even a minority of neurons fire in synchrony, a tone emerges that resembles the one that precedes radio Emergency Broadcast System announcements.
Like that tone, the neurons' synchronous signal calls attention to certain tasks -- and helps speed response time, according to a study in a recent issue of Nature by Robert Desimone, director of the McGovern Institute for Brain Research at MIT, and colleagues from the F. C. Donders Centre for Cognitive Neuroimaging at the Radboud University in Nijmegen, Netherlands.
The neurons studied belong to the V4 brain region, which plays an important role in activities involving visual attention, such as noticing when a traffic light changes. The two monkeys involved in this study had to notice a subtler change -- a white dot turning light yellow on a video screen. In some trials, the monkeys had to ignore similar but distracting lights in different parts of the visual field.
Previously, Desimone had observed that neurons harmonize their voices when monkeys pay attention. His new finding indicates that this synchronization speeds detection of important events.
"We selected an attention task that allowed us to determine the relationship between synchronized neurons and the ability to detect an event, on a trial by trial basis," said Desimone, who is also a professor in MIT's Department of Brain and Cognitive Sciences. "We found that for any given trial, the more coordinated the neurons, the faster the solution."
His lab measured the strength of the synchronous brain activity before, during and after the monkeys detected a change in the small light on screen. Just as drivers may initially miss the traffic light change while fiddling with the cell phone, the monkeys' attention may have wandered occasionally because the times to detect the light change varied significantly. About half a second before the change occurred, the researchers could predict from the amount of synchronized activity how fast the monkey would detect it. The weaker and less synchronized the brain signals, the slower the monkey's response.
The researchers also noted the diluting power of distraction. The more the monkeys' neurons synchronized on distracting lights, indicating they were not ignoring them as they should have, the more slowly the monkeys noticed the change in the actual target. A similar process might happen when a child with attention difficulty observes the birds outside the window instead of the teacher at the blackboard.
"Our findings suggest that the synchronization of V4 neurons reflects a general mechanism for rapidly funneling important information to other regions of the brain," Desimone said. "This leads us to ask if disruptions of neural synchrony might lead to some of the attention problems that are found in so many brain disorders."
The work was funded by the National Institutes of Health.