The ubiquitous nature of smartphones has raised concerns about their potential impact on brain health and mental well-being. This paper addresses the growing debate surrounding smartphone overuse and its potential addictive qualities. The sheer number of smartphone subscriptions globally—estimated at over six billion in 2022—underscores the device's pervasive influence on modern life. Smartphones are integral to communication, work, entertainment, and navigation, profoundly impacting nearly every aspect of daily existence. This widespread use has sparked considerable discussion regarding potential negative consequences, ranging from reduced productivity and academic achievement to increased negative emotionality and road safety risks. While correlational studies have linked excessive smartphone use to these negative outcomes, causal relationships remain unclear. It is crucial to differentiate between the medium (the smartphone) and its functions (apps, social media, etc.), as the specific content and needs fulfilled by these contents are key drivers of overuse. Research is needed to unravel the complex interplay of factors involved in smartphone overuse and its impact on mental health, cognitive functions and the brain.

A significant body of research, primarily utilizing self-report questionnaires, has investigated smartphone overuse through the lens of addiction. Scales like the Smartphone Addiction Scale (SAS) and the Korean Smartphone Addiction Proneness Scale (SAPS) are commonly employed. However, the appropriateness of the term "addiction" in this context is debated, with some researchers advocating for more neutral terms like "problematic smartphone use" or "Smartphone Use Disorder (SmUD)" to avoid overpathologizing everyday behavior. A recent meta-analysis reported alarmingly high global prevalence rates of smartphone addiction (up to 26.99%), particularly among adults. Several studies have attempted to delineate the specific contents driving excessive smartphone use, with social media applications being identified as a major contributing factor. The "data business model" of many social media apps, aimed at maximizing user engagement and data collection, further complicates the issue. Other factors implicated include video games and unregulated access to work emails.

This review focuses on neuroimaging studies, specifically those employing magnetic resonance imaging (MRI), to investigate the association between excessive smartphone use and variations in brain structure and function. The reviewed studies used various MRI techniques including voxel-based morphometry (VBM), diffusion tensor imaging (DTI), resting-state functional MRI (fMRI), and task-based fMRI. Structural MRI studies investigated grey and white matter alterations associated with SmUD tendencies, using both individual difference association designs and between-group designs. Functional MRI studies explored the intrinsic functional organization of the brain (resting-state fMRI) and neural activation during cognitive and affective tasks (task-based fMRI). Cue-reactivity paradigms, where participants are presented with smartphone stimuli, were also used. The studies reviewed employed a range of smartphone use assessment tools, adding to the heterogeneity of the results and complicating direct comparisons. Methodological inconsistencies included variations in MRI analysis strategies, a lack of standardized SmUD assessments, and differences in the rigor of brain analyses. Many studies also lacked sufficient statistical power and did not always adhere to appropriate multiple comparison correction procedures.

The review of structural MRI studies revealed inconsistent findings regarding gray matter volume differences associated with SmUD. Several studies reported decreased gray matter volume in regions such as the anterior cingulate cortex (ACC), orbitofrontal cortex (OFC), fusiform gyrus, parahippocampal regions, and the striatum, regions implicated in other substance and behavioral addictions. However, the lack of consistent replication across studies highlights the need for methodological standardization. Studies examining white matter tract integrity and cortical folding in the context of SmUD have also yielded mixed results. Functional MRI studies, predominantly using resting-state fMRI, revealed potential associations between SmUD tendencies and altered functional connectivity in various brain systems and networks, including striatal, limbic, and frontal regions, as well as alterations in the default mode network and the salience network. Task-based fMRI studies investigating cognitive and affective processes yielded limited and inconsistent findings, focusing on facial emotion processing, cue reactivity, and cognitive conflict tasks, indicating the need for more robust task-based fMRI studies. The findings often overlap with brain alterations observed in substance and behavioral addictions, suggesting shared neural mechanisms. However, the overall evidence remains insufficient to draw definitive conclusions about the effects of excessive smartphone use on brain structure and function due to inconsistencies in methodologies, small sample sizes, and cross-sectional study designs.

The current literature, while growing, offers insufficient evidence to definitively conclude the impact of excessive smartphone use on the brain. Methodological limitations, including a lack of standardized measures for assessing smartphone overuse, small sample sizes, and a predominance of cross-sectional designs, hinder robust conclusions. The absence of a consensus on the best assessment tools for smartphone overuse and the relative scarcity of studies incorporating objective, tracked smartphone usage data further complicate interpretation. While some studies found associations between excessive smartphone use and alterations in brain structure and function, it remains unclear whether these are causal relationships or merely reflect pre-existing predispositions. The findings highlight potential alterations in brain regions involved in reward processing, emotional regulation, and cognitive control, reminiscent of patterns observed in other addictive disorders. Future research must carefully address these methodological limitations to gain a clearer understanding of this complex relationship.

This review underscores the need for more rigorous research to determine the true extent of the brain changes associated with smartphone overuse. Future studies should prioritize larger samples, prospective longitudinal designs, and standardized assessment tools for SmUD. Integrating objective smartphone usage data through digital phenotyping will enhance the accuracy and interpretability of findings. Task-based fMRI studies focusing on key domains like cue reactivity, cognitive control, emotion processing and reward processing are needed. Further investigation into sex differences and the impact of smartphone overuse across the lifespan is also crucial. Adoption of transparent research practices, including data and code sharing and study preregistration, will significantly improve the replicability and reliability of findings in this field.

The main limitations of this review stem from the heterogeneity and methodological inconsistencies present in the existing neuroimaging literature on smartphone overuse. Many studies employed small sample sizes, lacked appropriate statistical power, and utilized diverse assessment tools, making it difficult to draw definitive conclusions. The majority of studies were cross-sectional, preventing any determination of causality. The lack of longitudinal studies means it is impossible to determine if brain changes are a cause or a consequence of excessive smartphone use.