August 29, 2012
Visual Awareness Improved Through Rhythmic Oscillations In Brain
redOrbit Staff & Wire Reports - Your Universe Online
Rhythmic oscillations in the brain provide heightened visual awareness that, when controlled, could help improve visual processing in critical environments such as air traffic control towers and nuclear power plants, according to a new study by the Beckman Institute.Using periodic visual stimuli and electroencephalogram (EEG) recordings, the researchers found they could precisely time the brain´s natural oscillations to future repetitions of an event. This effect occurred even after the prompting stimuli was discontinued.
This entrainment of brain oscillators can be used to lock the timing of repetitive brain activity and enhance “processing of subsequently predictable stimuli,” the researchers said.
“In nature, rhythmicity is everywhere, so it makes sense that our brain has evolved to be sensitive to rhythms in the world and to be able to latch on to them to improve neural processing,” said Beckman faculty member Alejandro Lleras, one of the researchers involved in the study.
“It´s very nice to be able to show that not only does the brain work in this oscillatory fashion but that we can harness that property that is inherent to the brain and use it to control the brain´s response.”
The current study follows a brain entrainment study conducted in 2010, in which a series of repetitive flashes were presented and followed by a faint target stimulus.
The scientists found that participants were only aware of those targets whose timing could be predicted based on the rhythm of the previous flashes. Targets presented on the offbeat were not seen.
“Awareness of near-threshold stimuli can be manipulated by entrainment to rhythmic events, supporting the functional role of induced oscillations in underlying cortical excitability, and suggest a plausible mechanism of temporal attention,” the researchers said.
Gabriele Gratton, one of the study´s researchers, said the idea in the current experiment was “to manipulate the brain activity and see if this manipulation was in fact predictive of performance for this phenomenon.”
Using EEG to test the theory, the researchers were able to assess the brain´s predictive responses, and demonstrate they could control them.
“We hooked up EEGs to measure the electrical activity from people´s brains to see if their brain waves were becoming locked to the rhythms, and they were,” said Kyle Mathewson, a Beckman fellow and the study´s first author.
“Then we showed that their visibility of the target fluctuated depending on the timing with respect to this rhythm. So we locked in the timing of their brainwaves and that locked in their ability to see the world at a certain time.”
The research line goes back to a discovery by Mathewson of a pulsed inhibition mechanism in the brain that is based on oscillations in the alpha phase (a relaxed state). This finding supported the theory that the brain sometimes samples the visual environment in rhythmic “frames” rather than continuously, as the term “brain waves” implies.
This study demonstrated that not only do these repetitive oscillations influence what we see in the world, but those momentary “snapshots” are controllable.
“We can actually line up the snapshots how we want them, so if we want the snapshot to be at a certain moment and not another, we can do that,” Mathewson said.
“Simply by exposing the brain to a predictable sequence of events, people were not only more likely to detect a faint target but we could see the brain oscillations shift to line up with the rhythmic sequence and target.”
Moreover, the entrained oscillations continued even after the visual stimuli ended, and the attention of test subjects still showed greater predictive awareness of future visual events when they were in time with the previous rhythm.
“When we stopped presenting the entrainers this repetitive brain activity continues,” Gratton said.
Mathewson said the results show that after the brain becomes attuned to the rhythmic stimuli, “it expects things to happen at that rhythm."
“So if you´re listening to a song at a certain rhythm you expect the next beat to come at a certain time,” he added.
“We present repetitive stimuli to participants and they start to predict the rhythm. Then we present the little flash of light and they are more likely to see it if it´s predictable, in time with the previous beat.”
The experiment used millisecond-long circle images as stimuli, and recorded the EEG signals of test subjects detecting the stimuli. The researchers said the results support the theory of a pulsed inhibition of visual processing.
“Rhythmic fluctuations in awareness elicited by entrainment of ongoing neural excitability cycles support a proposed role for alpha oscillations as a pulsed inhibition of cortical activity,” they wrote in a paper about the study published online last week in the Journal of Cognitive Neuroscience.
“This corresponds to our theory that changes in the ability to perceive stimuli that occur rhythmically are in fact determined by the oscillations in the brain,” Gratton said.
Beckman faculty member Monica Fabiani said the pulsed inhibition mechanism in the alpha phase is the brain´s way of making wise use of its resources.
“The system exploits the regularities in the environment to know when to pay attention and when not to,” she said. “Our brain is doing a lot of other stuff as well, so it´s advantageous to exploit the moment in which you do have to pay attention."
“Basically when you pay attention all the time your performance is equal. In the case of alpha oscillations on the other hand, you have really high performance when you need it and not much when you don´t need it. So it´s a more efficient system,” added Fabiani
“What happens is it´s not just that we pay more attention in this moment as it is we learn to pay less attention in that moment,” Gratton added. “So we learn to ignore moments that are not important, or not useful.”
The study has important implications, particularly for people in occupations where processing information is important.
“Alpha acts as a sensory inhibition mechanism that can reduce the processing of information, but only at particular phases in its cycle” the researchers wrote, while noting the opportunities inherent in discovering a way to “prep” the brain toward focusing visual attention.
“Here we provide a powerful technique to control these waves of consciousness, indicating that the brain is able to harness these perceptual snapshots of high excitability and optimal processing and align them with external events, a highly adaptive feature given the rhythmicity of our sensory world.”
Mathewson said the study could one day help researchers design human interfaces that harness the ability to control the timing of someone´s optimal attention to their visual world.
“This was a way that we were able to find out more about how the brain pays attention to the timing of things in the world,” he said.
“It´s really well-studied how we pay attention to a region of space compared to another region of space, but it´s very new to think about how we pay attention to this moment in time compared to the next moment," Mathewson added. “So we can use these regularities in the world and our brain can pick up on them to save a little bit of time and do a little bit better processing. If you had a pilot and you really wanted them to be aware of the warning signal, making its timing optimal is an important consideration.”
Fabiani noted that the study carries future applications for workers in critical environments, such as air traffic control systems or nuclear power plants.
“This could be instrumental for applied simulations like air traffic controllers, or other things that require constant vigilance, constant attention,” she said. “If you work to monitor the operator for example, you would know when a warning could be presented more efficiently to avoid disaster.”
The paper, entitled “Making Waves in the Stream of Consciousness: Entraining Oscillations in EEG Alpha and Fluctuations in Visual Awareness with Rhythmic Visual Stimulation,” was published August 20 in the Journal of Cognitive Neuroscience.