Temporal predictability enhances response inhibition in adults with attention-deficit/hyperactivity disorder (ADHD)

Individuals with Attention Deficit Hyperactivity Disorder (ADHD) are often characterised by impulsive behaviour. Here, we investigated whether temporal predictability can be used by adults with ADHD to help inhibit impulsive actions. Adults with ADHD and matched healthy controls performed a temporally cued Simon task in which target onset was either predictable or unpredictable. Targets appeared either on the same side as the responding hand (compatible trials) or on the opposite side (incompatible trials), with the latter requiring inhibition of prepotent responses. To characterise impulse control dynamics, we analysed Conditional Accuracy Functions (CAF), indexing the early activation of impulsive errors, and delta plots, capturing inhibitory efficiency over time. Temporal cueing increased the proportion of fast impulsive errors in both groups, indicating greater early response activation. Consistent with this increase in activation, temporal predictability sped responses to compatible targets in healthy controls. However, in adults with ADHD there was a tendency for temporal cueing to speed responses to incompatible rather than compatible targets. Delta plot analyses further clarified this RT pattern by revealing dissociable effects of temporal predictability on response inhibition across groups. In healthy controls, temporal predictability reduced inhibitory efficiency over time, whereas in adults with ADHD it improved inhibitory efficiency, resulting in more effective suppression of impulsive tendencies at longer response latencies. To better understand this effect, we examined individual differences in attentional function using a Continuous Performance Test (CPT). In ADHD, greater RT variability in a CPT—an index of attentional instability—was associated with larger cueing benefits for incompatible targets, suggesting that temporal predictability may scaffold performance when endogenous attentional control is weak. By contrast, higher commission errors in CPT predicted weaker cueing benefits in ADHD, suggesting that temporal predictability becomes less effective in individuals with high impulsive responding.