Post by Lincoln Tracy
What's the science?
Working memory refers to the temporary storage and manipulation of information. There is a wide-ranging network throughout our brain involving frontal and parietal regions that are responsible for working memory tasks. However, it is not known how the parietal brain regions can communicate with distant prefrontal brain regions (involved in cognitive control) in situations where a high level of cognitive function is required. Evidence from animal studies suggests that theta oscillations from the hippocampus—a pattern where neurons fire between five and 12 times per second—may play a role in working memory. This week in Nature Communications, Berger and colleagues explored the role of theta oscillations in the coordination of neuronal activity in remote brain areas in humans.
How did they do it?
The authors recruited 71 healthy volunteers (41 women) across four experiments to participate in a visual delayed match to sample task while wearing an electroencephalography (EEG) cap to record activity across different brain regions. For each trial of the task the volunteers were presented with a pattern of either one or four squares on a 6 x 6 grid on a computer screen for half a second. If the pattern was green the volunteers had to remember the pattern exactly as it was (retention). However, if the pattern was red, they had to remember a mirror image of the pattern (manipulation). After a two-second interval, the volunteers were presented with a second pattern and were required to indicate if the new pattern was the same as the previous one. First, the authors investigated how the accuracy of the task and frontal-midline theta activity varied according to trial complexity. The authors then investigated how the phase of the frontal-midline theta activity was related to fast rhythmic brain activity (an indicator of increased neuronal activity and information processing) in remote cortical areas. Finally, the authors used transcranial magnetic stimulation (TMS) during the interval between the presentation of the two patterns to examine the impact of disrupting fast rhythmic brain activity.
What did they find?
First, the authors found that the volunteers were more accurate when they only had to retain a less complicated pattern (i.e., a one-square pattern) compared to mentally manipulating a more complex pattern (i.e. the four-square pattern). Theta activity in frontal-midline brain region increased during trials involving the more complex pattern, regardless of whether participants had to remember or manipulate it. Second, they found that stronger gamma amplitude modulation by the frontal-midline theta phase occurred only in the right temporo-parietal brain region. They also found that the relationship between the frontal-midline theta phase and the gamma amplitude modulation in the right temporo-parietal region varied according to trial complexity. That is, as the trial became more complex, coupling with the gamma amplitude occurred during the trough of the theta cycle, as opposed to the peak of the theta cycle in the earliest condition. Finally, they found that the timing of the TMS pulse and the phase of the frontal midline brain region theta activity changed task performance. That is, TMS applied over the right temporo-parietal region just prior to the trough of the frontal-midline brain region theta activity resulted in poorer task performance. This finding was confirmed and replicated in the final two experiments.
What's the impact?
These findings suggest that the varying phases of the frontal-midline theta activity create windows in which wide-ranging neuronal activity can be synchronized during cognitively demanding tasks. In particular, these findings show that high-frequency EEG activity in the right temporo-parietal region is nested into frontal-midline theta activity and that the alignment of this nesting depends on the amount of cognitive focus when completing a working memory task. These findings highlight the importance of the coordination between brain oscillatory phases and neuronal firing on a wide scale. It is possible that the neural mechanisms identified in this study could serve as a general principle of how the brain coordinates parallel processes and dynamically allocates resources towards cognitively demanding tasks. Further research is required to determine if the proposed neural mechanism also exists for other frequencies and brain structures.
Berger et al. Dynamic regulation of interregional cortical communication by slow brain oscillations during working memory. Nature Communications (2019). Access the original scientific publication here.