Difficulty in Attention Switching and Its Neural Basis in Problematic Smartphone Use

<b>Background</b>: Problematic smartphone use (PSU) involves excessive smartphone engagement that disrupts daily functioning and is linked to attentional control deficits and altered reward processing. The nucleus accumbens (NAcc), a key structure in the reward system, may contribute to difficulty disengaging from rewarding digital content. This study examined relationships between NAcc volume, attentional switching, and objectively measured nighttime screen time in individuals with PSU. <b>Methods</b>: Fifty-three participants (aged ≥ 13 years) from an outpatient internet dependency clinic completed psychological assessments, brain MRI, and smartphone logging. PSU was diagnosed by two psychiatrists. Attentional switching was measured via the Autism Spectrum Quotient subscale. Nighttime screen time (00:00-06:00) was recorded via smartphone. MRI-derived NAcc volumes were normalized to total gray matter volume. Correlations, multiple regression (controlling for ASD and ADHD), and mediation analyses were conducted. <b>Results</b>: Difficulty in attention switching correlated with larger right NAcc volume (r = 0.45, <i>p</i> = 0.012) and increased nighttime screen time (r = 0.44, <i>p</i> = 0.014). Right NAcc volume also correlated with nighttime screen time (r = 0.46, <i>p</i> = 0.012). Regression showed right NAcc volume predicted nighttime screen time (β = 0.33, <i>p</i> = 0.022), whereas attentional switching was not significant. Mediation was unsupported. Sensitivity analyses confirmed associations. <b>Conclusions</b>: Larger right NAcc volume independently predicts prolonged nighttime smartphone use and is associated with impaired attentional switching in PSU. Structural variations in reward-related regions may underlie difficulty disengaging from digital content. Integrating neurobiological, cognitive, and behavioral measures offers a framework for understanding PSU.