Assessing the Longitudinal Relationship between Theta-Gamma Coupling and Working Memory Performance in Older Adults

The neural mechanism of non-phase-locked EEG activity in task switching

Flexible switching between different tasks is an important cognitive ability for humans and it is often studied using the task-switching paradigm. Although the neural mechanisms of task switching have been extensively explored in previous studies using event-related potentials techniques, the activity and process mechanisms of non-phase-locked electroencephalography (EEG) have rarely been revealed. For this reason, this paper discusses the processing of non-phase-locked EEG oscillations in task switching based on frequency-band delineation. First, the roles of each frequency band in local brain regions were summarized. In particular, during the proactive control process (the cue-stimulus interval), delta, theta, and alpha oscillations played more roles in the switch condition while beta played more roles in repeat task. In the reactive control process (post-target), delta, alpha, and beta are all related to sensorimotor function. Then, utilizing the functional connectivity (FC) method, delta connections in the frontotemporal regions and theta connections located in the parietal-to-occipital sites are involved in the preparatory period before task switching, while alpha connections located in the sensorimotor areas and beta connections located in the frontal-parietal cortex are involved in response inhibition. Finally, cross-frequency coupling (CFC) play an important role in working memory among different band oscillation. The present study shows that in addition to the processing mechanisms specific to each frequency band, there are some shared and interactive neural mechanism in task switching by using different analysis techniques.

Cognitive control, interference inhibition, and ordering of information during working memory in younger and older healthy adults

Investigating effects of aging on neurophysiological mechanisms underlying working memory provides a better understanding of potential targets for brain intervention to prevent cognitive decline. Theta-gamma coupling (TGC) indexes the ability to order information processed during working memory tasks. Frontal theta event-related synchronization (ERS) and parietal alpha event-related desynchronization (ERD) index cognitive control and interference inhibition, respectively. Relative contributions of TGC, theta ERS, and alpha ERD in relation to stimulus presentation are not characterized. Further, differential effect of normal aging on pre- or post-stimulus processes is unknown. Electroencephalography was recorded in 66 younger and 41 older healthy participants while performing 3-back working memory task. We assessed relationships between 3-back task performance and each of post-stimulus TGC, pre-stimulus parietal alpha ERD, and pre-stimulus frontal theta ERS in each age group. While older adults performed worse on 3-back task than younger adults, TGC, alpha ERD, or theta ERS did not differ between the two groups. TGC was positively associated with 3-back performance in both age groups; pre-stimulus alpha ERD was associated with performance among younger adults; and pre-stimulus theta ERS was not associated with performance in either group. Our findings suggest that both pre-stimulus interference inhibition and post-stimulus ordering of information are important for working memory in younger adults. In contrast, performance in older adults appears to depend only on post-stimulus ordering of information. These specific contributions of neurophysiological resources may explain the poorer performance of older adults and suggest different targets to enhance working memory in age groups.

Bridging the gap: TMS-EEG from Lab to Clinic

The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) has reached technological maturity and has been an object of significant scientific interest for over two decades. Ιn parallel, accumulating evidence highlights the potential of TMS-EEG as a useful tool in the field of clinical neurosciences. Nevertheless, its clinical utility has not yet been established, partly because technical and methodological limitations have created a gap between an evolving scientific tool and standard clinical practice. Here we review some of the identified gaps that still prevent TMS-EEG moving from science laboratories to clinical practice. The principal and partly overlapping gaps include: 1) complex and laborious application, 2) difficulty in obtaining high-quality signals, 3) suboptimal accuracy and reliability, and 4) insufficient understanding of the neurobiological substrate of the responses. All these four aspects need to be satisfactorily addressed for the method to become clinically applicable and enter the diagnostic and therapeutic arena. In the current review, we identify steps that might be taken to address these issues and discuss promising recent studies providing tools to aid bridging the gaps.