Trial-level sequence modeling reveals hidden dynamics of dual-task interference

Theories of dual-task interference assume that the same cognitive operations underlie multitasking regardless of stimulus timing, yet this core assumption has remained untested due to methodological limitations of behavioral averaging. Here, we combine hidden multivariate pattern (HMP) analysis with deep spatiotemporal sequence modeling of single-trial EEG to uncover the neural dynamics of multitasking in the psychological refractory period (PRP) paradigm. Using a deep spatiotemporal sequence model trained on Long stimulus-onset asynchrony (SOA) trials, we identify Encoding, Central, and Response operations and show that these same operations occur in the Short SOA condition, demonstrating shared cognitive processes across interference conditions. Additionally, trial-level decoding reveals multiple distinct sequences of cognitive operations across both tasks during interference, varying both within and across individuals. These sequences predict behavioral differences in reaction time and accuracy, revealing how interference timing within the cognitive operation sequence influences performance. In other words, we found trial-by-trial variability related to individual strategies directly affecting accuracy and reaction time (RT). Our findings challenge static bottleneck accounts and establish trial-level sequence modeling as a powerful tool to investigate the hidden dynamics of multitasking.Author summary: In our daily lives, we are often required to juggle multiple tasks at once, for example when operating an in-car device while driving. While multitasking is increasingly common, research shows that we often perform worse when multitasking, on average. Researchers have long been interested in when, how, and why this performance decline occurs, but most studies rely on averages that obscure how behavior changes at an individual trial level. In this paper, we use brain activity measured with electroencephalography (EEG) to identify recurring cognitive building blocks underlying simple decisions on individual trials. In the task we use, participants first perform a visual decision, followed by an auditory decision. The delay between the first and the second task was manipulated. When the delay is short enough, the tasks overlap and multitasking is more common. We find that the building blocks for performing both tasks remain the same when multitasking, which until now has only been assumed, but not directly demonstrated. Additionally, we find that how the building blocks of both tasks are ordered and combined varies across trials, and this variation predicts differences in response speed and accuracy for the first task. These findings show that multitasking behavior is more dynamic than traditionally assumed.