Wearing a KN95/FFP2 Facemask Has Subtle Effects on Functional Salience Network Interaction

Background Wide use of wearing a facemask is one of the many consequences of the COVID-19 pandemic. We used resting-state fMRI to search for subtle changes in brain functional connectivity, expected notably related to the high-level salience (SN) and default mode networks (DMN). Materials and Methods Prospective crossover design resting fMRI study with/without wearing a tight FFP2/KN95 facemask, including 24 consecutive male participants (mean age ± SD = 29.9 ± 6.9 years) at 3T. As physiological parameters, respiration frequency and heart rate were monitored. Data analysis was performed using the CONN toolbox. Results Wearing resulted frontal and precentral No difference was when or language were used as seed regions. cortices did signicant a facemask task-related fMRI sensorimotor We only very subtle changes related to wearing a facemask and those changes to occur in higher-level cognitive centers. The current resting-state fMRI study in 23 cases focusing on higher-level resting networks found signicant effects only related to the high-level SN. Of note, and consistent with a previous study 1 , our analysis did not reveal signicant differences using the lower-level sensorimotor or visual networks as seed regions.


Introduction
Wearing a facemask in professional settings was usually restricted to medical personnel, and surgical facemasks were typically used. The COVID-19 pandemic changed this reality, including the wearing of facemasks in daily life interactions and in MRI scanning facilities. Wearing a facemask on a daily basis is not supposed to interfere with cognitive abilities. Whether this practice impacts brain function remains unclear. To date, only one study in eight subjects indicated that it might alter BOLD baseline signals without affective task-related activation 1 . We used resting-state functional magnetic resonance imaging (fMRI) to assess subtle modi cations in functional connectivity using a crossover design, i.e., Half of the participants rst had no mask followed by mask, and half of the participants had the inverse order.
Since the behavioral consequences of wearing a facemask are minimal, we expected only very subtle changes in brain functional connectivity that should concern high-level brain networks rather than basic networks such as visual or motor networks. Consequently, we focused our analysis on the salience network (SN), which is involved in communication, social behavior, and self-awareness through the integration of sensory, emotional and cognitive information 2, and the default mode network, known for being active when a person is not focused on the outside world. To avoid potential bias of the resting fMRI results due to basic physiologic parameters, we monitored breathing and heart rate during the fMRI runs.

Participants
This prospective study was approved by the Ethics Committee of the University Hospitals and University of Geneva, Switzerland, the study was in accordance with the Declaration of Helsinki, and all participants gave written informed consent. The study included 24 community-dwelling cases recruited via advertisements in local media. One case was excluded from further analysis due to the incidental nding of an enlarged ventricular system. The nal sample included 23 cases (mean age ± SD = 29.9 ± 6.9 Heart rate and respiration data were collected continuously and simultaneously for resting-state functional imaging using BIOPAC systems (https://www.biopac.com/research/).
Data analysis was performed using the standard processing steps as described previously 3 . The data processing included motion correction, spatial ltering, denoising and transformation into NMI standard space using the individual 3DT1 brain sequence for anatomic spatial registration. Then, the functional time series were analyzed using the following steps: Fist, we performed a ROI-to-ROI analysis. The ROI-to-ROI connectivity metrics characterize the connectivity between all pairs of ROIs among a prede ned set of regions. The ROIs were de ned using the functional connectivity atlas included in the CONN toolbox. This atlas notably includes prede ned resting state networks of the SN, DMN, visual and language networks. This ROI-to-ROI analysis examines the functional connection between each pair of ROIs, for example DMN -SN. This pairwise ROI-to-ROI connectivity analysis was compared for FACEMASK versus no FACEMASK.
Second, we performed a seed-to-voxel analysis. This seed-based connectivity analysis aims to investigate functional properties from preselected seed regions. As seed regions, the same functional atlas-based regions were used as described above (SN, DMN, visual and language networks). Similar to the ROI-to-ROI analysis described above, a pairwise comparison for FACEMASK versus no FACEMASK was performed.
Both analyses (ROI-to-ROI and seed-to-voxel) therefore used the same prede ned atlas regions as ROI/seed. The difference between them is that the ROI-to-ROI only investigates functional connectivity between prede ned ROIs but cannot detect functional connectivity changes in other (nonprede ned) regions. In contrast, seed-to-voxel analysis is able to detect functional connectivity changes of the preselected seed ROI to any unselected voxel of the brain without reference to prede ned functional anatomic regions. Multiple-comparisons correction was applied using the false discovery rate (FDR) 4 .

Results
There were no differences in heart rate or respiration rate for FACEMASK versus NOMASK.
As a rst step, we performed an ROI-to-ROI analysis using prede ned atlas-based resting state networks, including the SN and DMN, as well as sensorimotor, visual and language networks. We found no signi cant direct effect of wearing a facemask within the established resting state networks, including the SN, DMN, sensorimotor, visual, dorsal attention or language networks.
As a second step, we performed a seed-to-voxel analysis using the networks mentioned above as seed regions. The resting fMRI seed-to-voxel analysis using the SN as the seed region revealed that FACEMASK versus NOMASK resulted in a signi cant reduction in a cluster in the left middle frontal and precentral gyrus (p<0.0002 FDR corrected), see gure Figure 1. There were no signi cant group differences using the DMN or sensorimotor, visual, dorsal attention or language networks as seed regions.

Discussion
The effect of wearing a tight FFP2/KN95 facemask on brain functional connectivity was signi cant using the SN but not the DMN, sensorimotor, visual, dorsal attention or language networks as seed regions. Importantly, there were no potentially confounding physiological effects since no group differences were found in heart rate or respiration.
The only previous study available in this eld 1 speci cally assessed the effect of wearing a facemask on functional MRI but with a different approach and objective. This study assessed task-related fMRI during a basic sensory-motor task designed to activate visual, auditory, and sensorimotor cortices in 8 participants. This study did not report signi cant effects of wearing a facemask on task-related fMRI activation in basic sensorimotor areas. We expected only very subtle changes related to wearing a facemask and those changes to occur in higher-level cognitive centers. The current resting-state fMRI study in 23 cases focusing on higher-level resting networks found signi cant effects only related to the high-level SN. Of note, and consistent with a previous study 1 , our analysis did not reveal signi cant differences using the lower-level sensorimotor or visual networks as seed regions.
We expected only very subtle changes related to wearing a facemask, and those changes to occur were related to higher-level cognitive centers. In agreement with our a priori hypothesis, the effect of facemask wearing was con ned to an interaction between the SN as the seed region and the left middle frontal and precentral gyrus. The SN is thought to facilitate the detection of important environmental stimuli 2,5 . It is mainly involved in detecting and ltering multimodal salient stimuli and mediating the transition from the DMN to the central executive network. Our results might imply that the wearing of a facemask might impact our ability to detect and react appropriately to salient stimuli by altering the functional connectivity of the SN. However, this phenomenon concerned only the connections to the left frontal and precentral gyrus. The signi cance of this nding remains unclear. Altered functional connectivity of the SN with frontal and precentral areas was reported as part of a more global cortical disconnection in a variety of neuropsychiatric disorders, including psychosis, poststroke depression and attention de cit syndrome 6-8 . In young controls, the connectivity between the SN and left frontal as well as precentral areas is thought to be crucial not only for episodic memory skills 9 but also for working memory activation 10 . Whether changes in restricted parts of SN functional connectivity may predispose individuals to decreased activation of these cognitive functions in highly demanding situations is still unknown.
Two limitations should be considered when interpreting the present ndings. To avoid the well documented gender-related differences in functional connectivity 11, we included only male participants in this study. Although this procedure makes sense given the small series of cases that accepted this constraining experimental design, it naturally limits the generalizability of the present ndings. Moreover, we deliberately used a tight FFP2/KN95 facemask, which many people prefer in the context of the COVID-19 pandemic and are requested by most airline companies. We would expect smaller effects on brain functional connectivity when using less tight surgical facemasks, which was the standard facemask in the eld of medicine before the COVID-19 pandemic. Future studies with various types of masks in mixed samples and using ad hoc activation paradigms will make it possible to complete these at rest observations, gaining better insight into the impact of facemask wearing on brain activation patterns.