Wearing a KN95/FFP2 facemask induces subtle yet significant brain functional connectivity modifications restricted to the salience network

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 magnetic resonance imaging (MRI) scanning facilities. This practice is widely accepted and well tolerated, thought to be effective for infection prevention [1].

In principle, wearing a facemask should not limit sensorial processing or high-level abilities such as thinking, reading, and writing. However, by impacting social interaction, facemasks modify the neural responses to recognition of facial cues but also pivotal human abilities serving our daily behavior such as emotion recognition, perceived closeness, trust attribution, and even re-identification of unmasked faces [2‐6]. Unlike behavioral paradigms, little is known about the effect of wearing a facemask on brain activation. The two available studies indicated that this practice has a subtle but still significant effect on cerebral blood flow and oxygen saturation but also blood oxygenation level dependent (BOLD) baseline signals [7, 8]. In a series of 13 young individuals wearing a FFP2 facemask, Fischer et al. [7] reported a 6.5% increase of cerebral blood flow and a 0.9% increase of oxygen saturation measured by transcranial hybrid near-infrared spectroscopy. In eight middle-aged subjects with a classical surgical facemask, Law et al. [8] indicated an altered BOLD baseline signals with a minimal impact on task-related activation.

To our knowledge, no previous study explored the effect of wearing facemask in at rest functional connectivity. We opted for focusing on two main networks involved in complex prosocial behaviors, the salience network (SN) and default mode network (DMN) [9]. Early studies showed that the patterns of functional connectivity of these networks are critically involved in various psychiatric disorders, including anxiety disorders, autism, schizophrenia, bipolar disorders, and dementias [10‐15]. More recently, the interplay between these networks has been involved in the successful performance of social-cognitive tasks toward a close other [16] as well as mentalizing performances [17].

In this study, we tested the hypothesis that wearing a facemask could induce changes in the at rest functional connectivity of these networks that may ultimately impact on our social interaction abilities.

There were no differences in heart rate or respiration rate between MASK and NOMASK conditions. The values provided by the MRI system BIOPAC are unitless values (for respiration frequency and pulse rate) and were for respiration 4.54 ± 1.55 (MASK) versus 4.88 ± 1.54 (NOMASK), p = 0.460, and for pulse 0.12 ± 0.01 (MASK) versus 0.12 ± 0.01 (NOMASK), p = 0.225.

We found no significant direct effect of wearing a facemask within the established resting state networks, including the SN, DMN, sensorimotor, visual, dorsal attention, or language networks. The resting fMRI seed-to-voxel analysis using the SN as the seed region revealed a significant reduction in a cluster in the left middle frontal and precentral gyrus for MASK versus NOMASK conditions (Fig. 1, Table 2).

The details of this significant cluster are illustrated in Fig. 2. The functional connectivity between the SN as seed region and this cluster was significantly (p-FDR < 0.001) lower for MASK versus NOMASK conditions. This effect was present for all seven subregions of the SN.

There were no significant group differences using the DMN or sensorimotor, visual, dorsal attention, or language networks as seed regions.

The present findings indicate that the effect of wearing a tight FFP2/KN95 facemask on brain functional connectivity was significant using the SN but not the DMN, sensorimotor, visual, dorsal attention, or language networks as seed regions. Basic physiological effect of altered breathing cannot be at the origin of the observed differences. This assumption is supported by two observations. First, we did not observe significant differences in the measured heart rate and respiration rate. Second, if the effect was a basic effect due to respiration, this effect should be present in all networks, including basic-level and high-level networks, but we observed a change in fMRI connectivity only in high-level SN.

The only previous study available in this field [8] assessed the effect of wearing a facemask on functional MRI but with different approach and objectives. This study assessed task-related fMRI during a basic sensory-motor task designed to activate visual, auditory, and sensorimotor cortices in eight middle-aged participants. The authors reported an altered BOLD baseline signals with no impact on task-related activation of sensorimotor areas. In our study, resting-state fMRI analysis in 23 cases focused on functional connectivity of higher-level resting networks. We also report no significant effect of wearing facemask on the functional connectivity of lower-level sensorimotor or visual networks. In agreement with our a priori hypothesis, the effect of this practice was confined to an interaction between the SN as the seed region and the left middle frontal and precentral gyrus. Although subtle, this effect concerned all of the SN subregions and was statistically significant.

The clinical significance of this finding 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 deficit syndrome [21‐23]. 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 [24] but also for working memory activation [25]. Our observations should be interpreted in conjunction with the recent report by Fischer et al. [7] who reported a subtle but still significant increase of cerebral blood flow and oxygen saturation when using the same type of facemask. Taken together, these observations suggest that wearing a FFP2/KN95 facemask may lead to compensatory increase of brain metabolism and deficits of functional connectivity in the SN. This latter is involved in the detection of environmental stimuli and mediates the transition from the DMN to the central executive network [9, 26]. Whether these changes are clinically silent or may predispose individuals to fail in highly demanding situations merits further consideration. Our observations should be viewed as a first step towards a more detailed analysis of facemask wearing repercussions in complex environments. In particular, the study of facemask wearing effect on brain activation in social interaction and mentalization paradigms, two main functions subserved by the SN, is a critical issue to address in order to get insight into the clinical significance of our work.

Some limitations should be considered when interpreting the present findings. To avoid the well-documented gender-related differences in functional connectivity [27], 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 findings. 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 field of medicine before the COVID-19 pandemic. Third, the presence of physiological noise could impact the quality of fMRI data. Last but not least, the changes in functional connectivity reported may vary as a function of the length of the period that subjects wear the masks. Since no changes in physiological parameters related to wearing a facemask were observed in this series, it is highly likely that the reported result is probably stable over time. However, the present experimental design does not allow for addressing this hypothesis.

Future studies with various types of masks in mixed samples including several time points of wearing facemask and using ad hoc activation paradigms that involves the SN will make it possible to complete these at rest observations, gaining better insight into the impact of facemask wearing on brain function.