Layers of waves
To conduct the study, Bastos worked from several lines of evidence and with some relatively new technology. Last year, for example, co-author and Picower Institute postdoctoral fellow Mikael Lundqvist led a study showing that gamma waves perked up in power when sensory (neuroscientists call it “bottom-up”) information was loaded into and read out from working memory. In previous work, Miller, Bastos and colleagues had found that alpha/beta rhythms appeared to carry “top-down” information about goals and plans within the cortex. Top-down information is what we use to make volitional decisions about what to think about, or how to act, Miller says.
The current study benefitted from newly improved multilayer electrode brain sensors that few groups have applied in cognitive, rather than sensory, areas of the cortex. Bastos realized that if he made those measurements, he and Miller could determine whether deep alpha/beta and superficial gamma might interact for volitional control of working memory work.
In the lab Bastos and co-authors including graduate students Roman Loonis and Simon Kornblith made multilayer measurements in six areas of the PFC as animals performed three different working memory tasks.
In different tasks, animals had to hold a picture in working memory to subsequently choose a picture that matched it. In another type of task, the animals had to remember the screen location of a briefly flashed dot. Overall, the tasks asked the subjects to store, process and then discard from working memory the appearance or the position of visual stimuli.
“Combining data across the tasks and the areas does lead to additional weight for the evidence,” Bastos says.
Across all the PFC areas and all tasks, the data showed the same thing: When sensory information was loaded into working memory, the gamma rhythms in superficial layers increased and the alpha/beta rhythms in deep layers that carried the top-down information decreased. Conversely, when deep-layer alpha/beta rhythms increased, superficial layer gamma waned. Subsequent statistical analysis suggested that gamma was being controlled by alpha and beta rhythms, rather than the other way around.
“This suggests mechanisms by which the top-down information needed for volitional control, carried by alpha/beta rhythms, can turn on and off the faucet of bottom-up sensory information, carried by gamma, that reaches working memory and is held in mind,” Miller says.
With these insights, the team has since worked to directly test this multilayer, multi-frequency model of working memory dynamics more explicitly, with results in press but not yet published.
Charles Schroeder, research scientist and section head in the Center for Biomedical Imaging and Neuromodulation at the Nathan S. Kline Institute for Psychiatric Research, describes two contributions of the study as empirically important.
“First, the paper clearly shows that critical cognitive operations (in this case working memory) are underlain by periodic (oscillatory) network activity patterns in the brain, and that these must be addressed by single trial analysis,” Schroeder says. “This provides an important conceptual alternative to the idea that working memory must involve continuous neural activation. Secondly, the findings strongly reinforce the notion that dynamic coupling across high and low frequency ranges performs a clear mechanistic function: lower frequency activity dominant in the lower layers of the prefrontal area network controls the temporal patterning of higher frequency information representation in the superficial layers of the same network of areas. The important conceptual innovation in this case lies in allowing lower frequency control operations to act directly on higher frequency information representation within each cortical area.”
Bastos says the model could be useful for generating hypotheses about clinical working memory deficits,. Aberrations of deep-layer beta rhythms, for example, could lead to a lessened ability to control working memory for goal- directed action.
“In a schizophrenia model or schizophrenia patients, is the interplay between beta and gamma lost?” he asks.
The National Institute of Mental Health and the Office of Naval Research provided funding for the study.
More about the image: Bastos acknowledges additional artistic contributions by Julien Vezoli and Pascal Fries, He adds: "The oscillations are inspired by a painting by synesthetic artist Sheila Rice, who experiences music and sounds as visual colors and waves. Her direct visual experiences of the back-and-forth solos between Miles Davis and Charles Mingus on the track "There's No You" (1968) were the basis for the waves depicted on the brain. Funding for the art project was provided by the Council for the Arts at MIT."
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