Smartphone behavior as a digital biomarker of brain function

Understanding how the brain operates in the context of the real-world is one of the main goals of neuroscience and smartphones may offer a path toward this aim. As ubiquitous, sensor-rich mobile devices, smartphones continuously and passively capture diverse real-world human behavior, from smartphone-derived behaviors (e.g, smartphone usage duration and touchscreen dynamics) to broader smartphone-inferred behaviors (e.g. social activity, daily routine, circadian rhythm, etc). Smartphones combined with neural measures have begun to reveal some amazing glimpses of brain functions. Evidence from neuroimaging, electrophysiological and behavior indicates that smartphone use is linked to structural and functional brain alterations across multiple neural processes, and the fluctuations in touchscreen interactions alone even serve as a sensitive indicator of brain following epileptic seizures. Together, these findings highlight the potential of smartphone behavior as a biomarker of brain function, analogous to heart-rate variability as an index of mental health and eye movements as indicators of neural information processing. Realizing this potential could enable continuous monitoring of brain health and support clinical applications such as early diagnosis, and recovery assessment. However, significant challenges remain, including measuring and quantifying smartphone behaviors alongside neural activity and establishing interpretable links between them. Longitudinal recording by pairing portable mobile imaging with smartphone behavior may help establish robust neuro-behavioral links, while experimental interventions, such as smartphone deprivation or behavioral modulation, could clarify causality. Additionally, dimensionality reduction approaches may further identify key neural features underlying specific smartphone behaviors. Ultimately, overcoming these challenges could enable smartphones to evolve from passive observers of daily behavior to active indicators of brain function, advancing a more integrated and ecologically grounded neuroscience of everyday behavior.