Brain development is a dynamic, non-linear process influenced by genetic and environmental factors. Environmental influences include relationships and experiences and can be nurturing, adverse or neutral. Given the prominent and increasing role of digital media for families beginning in infancy, it is critical to understand the direct and indirect impacts of various aspects of use on emerging skills and underlying neurobiology. These are likely to be greatest during early childhood when brain networks develop rapidly and plasticity is high, manifest via differences in gray and white matter structure.30 The purpose of this study was to examine associations between digital media use and established measures of cortical morphology (CT, SD) at this formative age. In line with our hypotheses, higher use was related to decreased CT and SD in both primary visual and higher-order association areas.
Cortical thickness (CT) reflects synaptic density and supporting cellular architecture.49 While overall CT reaches maximal levels by age 2, that of limbic and sensory areas precedes higher-order (e.g. association, executive) areas, which do not achieve local maxima until adolescence.35 Changes reflect cortical remodeling in response to environmental stimulation, which can be accretive (e.g., synaptogenesis) or reductive (e.g., pruning).49 The current study involved 3-5-year old children, whose overall CT is expected to have largely peaked, though not in higher-order areas. While threshold-free maps suggest lower CT related to higher ScreenQ scores in diffuse, bilateral brain regions (Fig. 1s), these were lateralized to the right hemisphere, including all clusters reaching statistical significance. Significant clusters were in occipital and parietal areas (Fig. 1) that support both sensory (e.g., primary visual) and higher-order associative (e.g., supramarginal gyrus) processes, suggesting impacts on areas expected to be mature at this age and others that are still developing.
Synchronous thinning in functionally related areas has been linked to environmental factors (e.g., visual network, visual stimuli).42 Thinning in visual cortices has also been attributed to higher maturation and efficiency.7 Association between higher ScreenQ scores and lower CT in occipital areas in the current study is consistent with these models, likely via greater exposure to screen-based media during early childhood. Higher ScreenQ scores were also associated with lower CT in the right superior parietal lobe, which is a major node in the “top-down” dorsal attention network, particularly involving visual-spatial stimuli.50
Association between higher ScreenQ scores and lower CT in the postcentral gyrus, whose major role is somatosensory processing, is more counter-intuitive. A reasonable mechanism involves the stimulation of mirror neurons during the processing of imagined sensations in video scenes.51,52 Indeed, these clusters with lower CT were in the more posterior Brodmann Area 2, where mirror neurons are well-documented53 and which supports higher-order somatosensory processing.54 Thus, if this mechanism is accurate, a major question is whether somatosensory cortical remodeling via digitally presented scenes is of functional relevance compared to thinning that may manifest via real-world situations.
In contrast to sensory areas where thinning is generally adaptive, CT in higher-order areas (e.g., executive, association) has been positively associated with cognitive performance, including IQ, language and emergent literacy skills.36–38 Thus, it is less clear whether associations between higher ScreenQ scores and lower CT in the right inferior parietal lobe, which supports multi-modal (e.g., visual, somatosensory, emotional) processing55 and also learned and creative skills such as music56 and math,57 are benign or maladaptive in nature. Similarly, higher media use was associated with lower CT in the right supramarginal gyrus (SMG), a higher-order area not expected to have peaked at preschool age. The right SMG supports empathy (in children, overcoming egocentricity bias),58,59 and lower CT in this area has been linked to conduct disorder in adolescents.60 While not assessed here, excessive and inappropriate digital media use has been linked to lower empathy,61 and a “video-deficit” in social cognition described in preschool-age children.62 Thus, while speculative, findings in the current study may reflect SMG under-development at this age, a potential early biomarker of impacts of higher media use on social cognition. Interestingly, the postcentral gyrus is also involved with emotional processing and empathy (largely via the mirror neuron system), with lower CT possibly linked to these domains.54 Further studies involving measures of social cognition and other skills would be helpful to better characterize these potential impacts.
The current findings align with those from the large, ongoing ABCD study involving pre-adolescent children, where higher media use was associated with lower CT in both sensory (including primary visual, postcentral) and higher-order (including SMG) areas.7 The authors attributed these findings to accelerated maturation of the visual system, with impacts on other, non-functionally homologous areas less clear. At a minimum, findings in the current study involving visual areas are highly consistent, suggesting that relationships between higher media use and brain structure begin to manifest in early childhood and become more extensive over time. They are also consistent with a recent functional MRI study involving preschool-age children, where functional connectivity involving primary visual networks was maximal during animated relative to traditional story formats, a potential mechanism for accelerated thinning.63
Sulcal depth (SD) is an established measure of cortical surface area, which exhibits more gradual maturational changes with age, reaching overall maxima in late childhood.35,49,64 The current study found significantly lower SD in two clusters in the right inferior temporal/fusiform gyrus, which supports processing specific categories of visual stimuli (e.g., places, shapes).65,66 One interpretation of this finding, akin to that for lower CT in occipital areas, is accelerated maturation of visual areas via higher media use. However, the fusiform cortex includes the putative Visual Word Form Area (VWFA), which develops to rapidly process letters and words during reading.67 Thicker fusiform cortex has been associated with higher reading abilities in children,41 including at young ages before formal reading instruction.68 The current findings complement those of thicker fusiform cortex in preschool-age children with higher emergent literacy skills.43 They also align with associations between higher media use (ScreenQ) and both lower emergent literacy skills and measures of white matter microstructure supporting these skills found in a related study involving preschool-age children.26 Thus, while speculative, the current findings may be a biomarker of impacts of higher digital media use on cortical surface area (SD) supporting reading at this age, though further studies are needed.
This study has limitations that should be noted. While 17% of participants met poverty criteria, the sample was largely of higher income and maternal education, and results might be different with greater socioeconomic diversity. Analyses were limited to children completing MRI and meeting necessary motion criteria, which may bias results towards those with higher self-regulation and other behavioral characteristics. Findings of lower CT did not survive at FWE-p < 0.05 yet did for FWE-p < 0.10, yet stringent correction greatly reduces the likelihood of false positives and findings for SD survived both cutoffs. The cross-sectional nature prohibits comment on causality, which requires a longitudinal design. It is also impossible to discern whether associations between higher use and lower CT and SD stemmed from direct (e.g., visual stimulation) or indirect (e.g., displacement of reading) mechanisms. While differences in cortical morphology related to higher use were found at a single time point, rates of change may be more relevant to cognitive development.69 Finally, while there were structural differences in areas known to support higher-order skills (notably social cognition), measures of these were not administered, rendering brain-behavior relationships speculative.
This study also has important strengths. It involves a reasonably large sample of very young children, where there have been few MRI-based studies involving media use and none of these to our knowledge involving cortical structure. Analyses controlled for age and sex, which may reflect general maturation rather than environment.34,70,71 Findings align with the ABCD study involving adolescents,7 and complement previous studies at this age involving differences in cognitive skills, functional connectivity and white matter microstructure.26,43,63 Findings involved both CT and SD, complimentary measures with non-uniform developmental trajectories, reflecting synapse-level changes and brain growth.35 Altogether, while several findings are unclear, attributable to the complex nature of cortical development, this study provides novel evidence that changes related to digital media use are evident in early childhood. Longitudinal studies, ideally beginning in infancy given trends in digital media use, are needed to characterize longer-term impacts on cognitive, social-emotional and overall health outcomes.