Executive Function in the Toddler Data Desert: Working Memory Search and ADHD Risk in a Touchscreen fNIRS Paradigm

Attention-Deficit/Hyperactivity Disorder (ADHD) is associated with early differences in attentional control and working memory (WM), yet few paradigms can isolate spatial versus featural attention mechanisms during the toddler-to-preschool period. Addressing this “Toddler Data Desert,” we developed a touchscreen WM search task paired with functional near-infrared spectroscopy (fNIRS) to characterize developmental change and early neural markers of ADHD risk. Methods. Participants were 190 children ages 2–6 years (M = 4.3; 86 at risk for ADHD based on SDQ Hyperactivity cutoff ≥7). Children completed the Treasure Chest Task, a self-ordered pointing WM search paradigm with stationary (spatially informative) and scrambled (featural/color informative) conditions across set sizes 3 and 6 (and set size 9 for a subset). fNIRS recorded hemodynamic responses across bilateral frontal, temporal, and parietal regions. Mixed-effects models tested effects of age, condition, set size, and ADHD risk. Results.Behaviorally, children required fewer selections on spatially informative trials than featural trials at set sizes 3 and 6, with performance improving with age. Under super-capacity demand (set size 9), spatial–featural differences diminished and variability increased. Perseveration decreased robustly with age, indicating developmental gains in cognitive control and strategy updating. ADHD risk did not significantly predict behavioral efficiency. Neurally, spatial blocks showed age-related increases in frontal recruitment, consistent with a shift toward top-down control. During featural blocks, children engaged a left-lateralized frontotemporal-parietal network, while ADHD-risk children exhibited broader frontal recruitment, including additional right frontal engagement. Conclusions. This toddler-feasible touchscreen fNIRS paradigm dissociates spatial and featural attention within WM search and reveals early neural signatures of ADHD risk despite comparable behavioral performance.Keywords: fNIRS; working memory; dimensional attention; ADHD risk; early childhood