Magnetoencephalography and electroencephalography (M/EEG) seed-based connectivity analysis requires the extraction of measures from regions of interest (ROI). M/EEG ROI-derived source activity can be treated in different ways. It is possible, for instance, to average each ROI's time series prior to calculating connectivity measures. Alternatively, one can compute connectivity maps for each element of the ROI prior to dimensionality reduction to obtain a single map. The impact of these different strategies on connectivity results is still unclear. Here, we address this question within a large MEG resting state cohort (N=113) and within simulated data. We consider 68 ROIs (Desikan-Kiliany atlas), two measures of connectivity (phase locking value-PLV, and its imaginary counterpart- ciPLV), and three frequency bands (theta 4-8 Hz, alpha 9-12 Hz, beta 15-30 Hz). We compare four extraction methods: (i) mean, or (ii) PCA of the activity within the seed or ROI before computing connectivity, map of the (iii) average, or (iv) maximum connectivity after computing connectivity for each element of the seed. Hierarchical clustering is then applied to compare connectivity outputs across multiple strategies, followed by direct contrasts across extraction methods. Finally, the results are validated by using a set of realistic simulations. We show that ROI-based connectivity maps vary remarkably across strategies in terms of connectivity magnitude and spatial distribution. Dimensionality reduction procedures conducted after computing connectivity are more similar to each-other, while PCA before approach is the most dissimilar to other approaches. Although differences across methods are consistent across frequency bands, they are influenced by the connectivity metric and ROI size. Greater differences were observed for ciPLV than PLV, and in larger ROIs. Realistic simulations confirmed that after aggregation procedures are generally more accurate but have lower specificity (higher rate of false positive connections). Though computationally demanding, after dimensionality reduction strategies should be preferred when higher sensitivity is desired. Given the remarkable differences across aggregation procedures, caution is warranted in comparing results across studies applying different methods.

Transcranial alternating current stimulation (tACS) mechanisms remain poorly understood. While tACS is expected to modulate periodic electroencephalography (EEG) activity due to the imposition of oscillatory alternating currents, it may also modulate the aperiodic EEG component, suggesting alternative tACS mechanisms that have not yet been explored. This study examined whether 20-minute tACS session delivered at 40 Hz over C3/C4 can selectively modulate periodic and aperiodic components. We implemented a paired, counterbalanced, and sham-controlled design consisting of two sessions during which tACS was administered over the sensorimotor regions. EEG was recorded before and after tACS under three EEG conditions: resting state, active task (i.e., the serial reaction time task), and passive task (i.e., 40-Hz auditory stimulation). We found that tACS led to selective modulation of the aperiodic component, specifically a broadband downward shift in the power spectrum (i.e., reduction in the offset), independent of the EEG conditions and with no evidence of an effect on periodic activity. This result may reflect a general effect on neural spiking rather than direct modulation of oscillatory activity. Disentangling the contribution of the aperiodic component from periodic activity represents an opportunity to identify alternative neurophysiological mechanisms underlying neural modulation and provides a clearer interpretation of tACS mechanisms.

Objective: People who have survived stroke may have motor and cognitive impairments. High dose of motor rehabilitation was found to provide clinically relevant improvement to upper limb (UL) motor function. Besides, mounting evidence suggests that clinical, neural, and neurophysiological features are associated with spontaneous recovery. However, the association between these features and rehabilitation-induced, rather than spontaneous, recovery has never been fully investigated. The objective was to explore the association between rehabilitation dose and UL motor outcome after stroke, as well as to identify which variables can be considered potential candidate predictors of motor recovery. Methods: People who survived stroke were assessed before and after a period of rehabilitation using motor, cognitive, neuroanatomical, and neurophysiological measures. We investigated the association between dose of rehabilitation and UL response (ie, Fugl-Meyer Assessment for upper extremity [FMA-UE]), using ordinary least squares regression as the primary analysis. To obtain unbiased estimates, adjusting covariates were selected using a directed acyclic graph. Results: Baseline FMA-UE was the only factor associated with motor recovery (b = 0.99; 95% CI = 0.83 to 1.15 points). Attention emerged as a confounder of the association between rehabilitation and final FMA-UE (b = 5.5; 95% CI = −0.8 to 11.9 points), influencing both rehabilitation and UL response. Conclusion: Preserved attention in people who have survived stroke might lead to greater UL motor recovery, albeit estimates have high levels of variability. Moreover, the increase in the dose of rehabilitation can lead to 5.5 points improvement on the FMA-UE, a nonsignificant but potentially meaningful finding. The approach described here discloses a new framework for investigating the effect of rehabilitation treatment as a potential driver of recovery. Impact: Attentional resources could play a key role in UL motor recovery. There is a potential association between amount of UL recovery and dose of rehabilitation delivered, needing further exploration. Preserved attention and rehabilitation dose are candidate predictors of UL motor recovery.

Background. The left and right temporoparietal junctions (TPJs) are two brain areas involved in several brain networks, largely studied for their diverse roles, from attentional orientation to theory of mind and, recently, predictive processing. In predictive processing, one crucial concept is prior precision, that is, the reliability of the predictions of incoming stimuli. This has been linked with modulations of alpha power as measured with electrophysiological techniques, but TPJs have seldom been studied in this framework. Methods. The present article investigates, using magnetoencephalography, whether spontaneous oscillations in pre-stimulus alpha power in the left and right TPJs can modulate brain responses during a linguistic task that requires predictive processing in literal and non-literal sentences. Results. Overall, results show that pre-stimulus alpha power in the rTPJ was associated with post-stimulus responses only in the left superior temporal gyrus, while lTPJ pre-stimulus alpha power was associated with post-stimulus activity in Broca’s area, left middle temporal gyrus, and left superior temporal gyrus. Conclusions. We conclude that both the right and left TPJs have a role in linguistic prediction, involving a network of core language regions, with differences across brain areas and linguistic conditions that can be parsimoniously explained in the context of predictive processing.

Motor inhibition is the ability to suppress inappropriate motor responses. Dual-site transcranial magnetic stimulation studies suggest that the right inferior frontal gyrus (rIFG) can inhibit the primary motor cortex (M1), but it remains unclear whether inducing plasticity along this pathway affects motor inhibition. To address this issue, we used cortico-cortical paired associative stimulation (ccPAS) in healthy participants and assessed behavioral and electrophysiological markers of inhibition. In two sessions, we administered a ccPAS protocol to boost rIFG-M1 connections via Hebbian-like plasticity (ccPAS-rIFG-M1) and a reverse-order protocol (ccPAS-M1-rIFG) as a control. Consistent with well-documented inter-individual variability in response to brain stimulation, we divided participants into higher- and lower-sensitivity groups based on the progressive increase in motor excitability during ccPAS-rIFG-M1. Behavioral performance in the Go-NoGo task did not change as a function of ccPAS. However, independently of the ccPAS protocol, higher-sensitivity participants maintained an efficient proactive inhibitory strategy over time, whereas lower-sensitivity participants displayed increased response readiness at the expense of inhibitory control, suggesting stable individual differences rather than direct effects of stimulation. Furthermore, electrophysiological findings revealed that, following ccPAS-rIFG-M1, higher-sensitivity individuals showed a state-dependent increase of rIFG inhibitory influence over M1 only when viewing NoGo-cues. No comparable modulation was found for Go-cues, in lower-sensitivity individuals, or after ccPAS-M1-rIFG. These findings indicate that ccPAS-rIFG-M1 selectively modulates rIFG-M1 inhibitory interactions in a state- and sensitivity-specific manner, supporting a specific involvement of the targeted circuit in motor inhibition and linking individual plasticity capacity to stable differences in proactive inhibitory control.