Distinct timescales dissociate spontaneous thought dimensions.

Our spontaneous thoughts encompass various dimensions, such as task-relatedness (off vs. on task), and thought orientation (internal vs. external). However, their distinction remains unclear. Our study addresses this issue by focusing on their timescales at both the behavioral level (using fast and slow finger tapping) and the neural level (using EEG) using two independent datasets (N = 84 and 35). Behavioral results revealed a double dissociation: Task-relatedness was linked to fast tapping only, whereas thought orientation was associated with slow tapping only. At the neural level, we assessed topographic similarity in EEG to quantify the temporal influence of past neural activity on current ones. Task-relatedness modulated topographic similarity only during fast tapping, while thought orientation did so only during slow tapping. Critically, topographic similarity was phase-based, as shown by its correlation with phase-locking value and the loss of associations with thought after phase-shuffling. This indicates that the neural signatures of both thought dimensions are strongly phase-dependent. Finally, we demonstrate that nonlinearity plays a distinct role mediating the impact of different timescales (slow and fast finger tapping) on spontaneous thoughts at both behavioral (precision error) and neural levels (topographic similarity). Overall, these results demonstrate that task-relatedness is associated with short timescales, whereas thought orientation is associated with long timescales. This highlights how distinct temporal dynamics shape different spontaneous thought dimensions on both their behavioral and neural features.