Probing Executive Function in the Toddler Data Desert: A Touchscreen fNIRS Study of Working Memory and ADHD-Related Symptoms

Attention-deficit/hyperactivity disorder (ADHD) is associated with early differences in attentional control and working memory (WM), yet relatively few paradigms are developmentally feasible across the toddler-to-preschool period while also distinguishing spatial from featural information use. Addressing this "Toddler Data Desert," we extended a self-ordered pointing paradigm into a touchscreen WM search task paired with functional near-infrared spectroscopy (fNIRS) to characterize developmental changes in search behavior and cortical recruitment. The larger study enrolled 204 children aged 2-6 years; 200 completed both Set Size 3 conditions, 197 completed both Set Size 6 conditions, and 162 completed both Set Size 9 conditions after a performance-based advancement criterion. Children completed stationary, spatially informative and feature-informative, featural/color-informative Treasure Chest blocks across increasing memory loads while fNIRS measured bilateral frontal, temporal, and parietal responses. Behaviorally, children required fewer selections during spatial than featural search at Set Sizes 3 and 6, and performance improved with age; this condition difference was no longer statistically reliable at Set Size 9 within the selected higher-performing subset. Immediate perseverative selections also decreased markedly with age. Exploratory post hoc sequence analyses further showed increasingly organized spatial search under higher memory load, whereas featural search remained comparatively heterogeneous, and organization in a preceding spatial block did not reliably predict subsequent featural search. Symptom-defined ADHD risk did not significantly predict behavioral efficiency. Neurally, spatial and featural blocks showed age-associated differences in distributed frontotemporal-parietal recruitment, and the elevated-symptom group showed broader frontal recruitment despite comparable behavioral performance. Together, these findings support the Treasure Chest Task as a toddler-feasible approach for examining developmental differences in spatial and featural working-memory search while highlighting the value of integrating behavioral sequence measures with cortical activity. Neural risk-group findings should be interpreted as cross-sectional associations rather than diagnostic biomarkers.