Dissociating stimulus and task effects on neural oscillations during n-back tasks

N-back tasks are widely used to assess working memory functioning, yet different methodological approaches between tasks and across laboratories can yield distinct neural signatures. While working memory-related event-related potentials (ERPs) have been extensively studied, it remains unclear how task parameters influence the neural oscillatory activity underlying working memory functioning.

We conducted time-frequency analyses on EEG data from 36 participants performing nine different variants of the n-back task that systematically varied in stimulus type (words, pictures, colors) and task structure (timing, response, and feedback parameters). We examined power modulations in the theta (4-8 Hz), alpha (8-12 Hz), and beta (12-30 Hz) frequency bands to determine which task parameters drive changes in specific neural oscillations.

Results revealed a functional double dissociation: frontal theta event-related synchronization was selectively sensitive to stimulus type, with 'pictures' eliciting greater theta power than words and colors in the task variant with the shortest total trial duration and a requirement to respond to all stimuli, but not in other task variants. In contrast, centro-parietal alpha event-related desynchronization was driven predominantly by task structure rather than stimulus content, showing reduced alpha suppression in the task variant characterized by the longest inter-trial interval and immediate accuracy feedback. Beta oscillations showed no systematic modulation at the Region of Interest level. Cluster-based permutation analyses further confirmed the early temporal specificity of both theta and alpha effects.

We conclude that theta oscillations reflect stimulus-driven encoding demands, while alpha desynchronization tracks the structural and temporal demands of the task context (modulating the need for active neural inhibition of task-irrelevant information). These findings have direct implications for standardizing n-back protocols: stimulus type and task structure exert their influence on fundamentally different oscillatory systems and must be carefully controlled depending on which neural biomarker is targeted.