Networks in the infant's brain develop considerably during infancy. Although directly after birth, many of the macroscopic structures and connections are already present, during the first year of life, the brain network optimizes by pruning unimportant and strengthening important connections (Fair et al., 2009; Keunen, Counsell, & Benders, 2017). Current theories highlight that experience is essential for this brain network optimization. Neuroconstructivism explains neural development as an ongoing interaction between neural activity, gene expression, and the environment (Westermann et al., 2007). During development, neural activity, either spontaneously generated or derived from sensory experience, plays an integral part in shaping neural networks. These changes to the brain change cognitive behavior, leading to new experiences and changes in the brain. Therefore, the development of brain and behavior is heavily intertwined. The development of social competency is a noteworthy example of a type of cognitive change inextricably linked to both the development of the brain and the increases in available new experiences. The brain optimizes through the processing of social behavior, which leads to more complex social cognition, increasing social learning possibilities. The human ability to learn from others has been deemed crucial for human evolutionary success (Tomasello, Kruger, & Ratner, 1993) and human infants show considerable social learning capabilities during infancy (Clearfield, Osborne, & Mullen, 2008; Elsner, 2007). These social learning situations create new experiences, which change the brain even further. Note, that these new experiences need not be strictly social. Therefore, developing social competency could influence the brain as a whole. Thus, to develop the infant brain network, new social experiences are critical. This is most critically shown in the relationship between infant and caregiver. For example, synchronous mother-child interactions have been shown to facilitate emotional, cognitive, and social growth (Harrist & Waugh, 2002). Opposingly, the lack of a caregiver removes the possibility of some new social experiences, influencing both neural and behavioral development negatively (Cirulli, Berry, & Alleva, 2003; Faturi et al., 2010; Kaufman, Plotsky, Nemeroff, & Charney, 2000). While within-family social interaction is the most important social bond during infancy. Infants also get extensive amounts of social cues from family friends, daycare, and other random interactions. These interactions have been severely hampered in the past years due to COVID-19 and its concurrent regulations. The closing of schools and daycare, wearing masks, keeping distance, and a general limiting of unnecessary social interaction severely limited the available social cues for newly born infants. It is unlikely that this has lowered the number of social interactions of an infant - parents worked from home more often and infants still needed to be cared for, but the number of interactions with different persons and with it the heterogeneity of social cues was likely affected. Studying the effects of COVID-19 regulations on brain development sheds light on the importance of the heterogeneity of these social interactions during the first year of life. Typical maturation of the infant brain network Structurally, most of the macrostructures of the infant brain network are already present at birth (Ball et al., 2014; van den Heuvel et al., 2014). The neonate brain network is, however, still unoptimized. Therefore, the brain spends the first years of life on optimization, through the selective pruning of rarely used connections and the strengthening of oft-used connections (Keunen et al., 2017). Functionally, a similar strive for optimization has been shown. Activity patterns in distinct brain areas become more focal and stronger during development, indicating an increase in specialization (Durston et al., 2006). This coincides with increased integration of brain information, through increased reliance on long-range connections in the developing connectome (Fair et al., 2009). It is common to use a graph-theoretical framework, to calculate this optimization of the brain network. In graph theory, networks consist of nodes (the centers of information) and edges (the connections between the nodes). A network structure consisting of many clusters and a relative ease of going from one part of the network to another is generally seen as an optimal network since it combines two key features of any network: specialization and integration. This optimality is captured in the small-worldness index (Watts & Strogatz, 1998). While neonate brain networks do show some semblance of small-worldness (Fransson et al., 2007), many studies have found evidence for further optimization during childhood. For example, small-worldness propensity increases between 5 and 10 months old (van der Velde, 2022). Similar developments of the optimization of the brain network have been found in children between 2 and 6 years old (Bathelt, O'Reilly, Clayden, Cross, & de Haan, 2013) and in children between 5 and 7 years old (Boersma et al., 2011). It is therefore apparent that this optimization process is gradual and constant throughout the early years of life. In addition to the optimization of the network, maturing infant brain networks also show increases in general functional connectivity (Gao, Lin, Grewen, & Gilmore, 2016; Xie, Mallin, & Richards, 2018). Lastly, a reorganization of the functional infant brain network occurs somewhere in the second half of the first year of life. Previous work by our lab showed that EEG theta networks, but not alpha networks, showed a reorganization from a parieto-occipital network towards a frontoparietal network. After this change in functional network topology, the networks became selective for social versus non-social stimuli, showing increased functional connectivity when social stimuli were processed. Therefore, this change might represent the switch from the brain trying to visually describe a stimulus to the brain trying to interpret this stimulus (van der Velde, White, & Kemner, 2021). The current study Taken together, infants experienced a less heterogeneous environment of social cues during the COVID-19 pandemic while the brain uses these cues to optimize throughout the first year of life. Previous work tells us that caregiver-infant interactions are vital for developing cognitive and brain functioning. However, we do not know the effects of the heterogeneity of these cues on social and brain functioning. Gaining insight into this might help us create better environments for infants to grow up in, but also could be used to recognize how drastic these regulations truly are in case of future unforeseen events. Therefore, this study aimed to examine the relationship between COVID-19 regulations and brain network development, with a longitudinal framework, during the first year of life. To this end, we compared functional EEG brain networks of 150 infants born after the COVID-19 regulations started with 150 infants born and tested before COVID-19. Both groups of infants were tested twice, around 5 and 10 months of age. The following properties of the infant brain networks were compared: 1) optimization, 2) average network strength, and 3) frontoparietal connectivity. We studied theta and alpha networks, due to their reliability and possible involvement in social processing (van der Velde et al., 2021; van der Velde, Haartsen, & Kemner, 2019). We expected infants born and tested after COVID-19 regulation began to have lower theta and alpha network optimization both at 5 and 10 months old and show a slower development of network optimization over time. Similar results are expected for global network strength. Lastly, we expect lower theta frontoparietal connectivity in 10-months-old infants, but not in 5-months-old infants, since theta brain network reorganization takes place between 5 and 10-months-old.