Spontaneous Mind-Wandering Thought Content is Less Constrained Than Controlled Stimulus-Independent Thought.

Achieving a complete understanding of our internal thought processes requires us to understand the behavioral and neural distinctions underpinning controlled thoughts (stimulus-independent control) and spontaneous thoughts (mind-wandering). One unexplored question is whether thought content differs with differing levels of thought control. We had participants perform a task known to elicit high levels of mind-wandering. Throughout the task, we asked participants to report the content of their thoughts in their own words and identify whether their thoughts were externally or self-generated and whether their thoughts were under their control. Task-unrelated thought content was more heterogeneous within participants compared to other thought categories. Self-generated spontaneous thoughts were less similar than self-generated controlled thoughts, even when analyses were restricted to task-unrelated thoughts. Examination of thought content using sentence embeddings found thought content naturally clustered based on thought category. Finally, we found the number of task-unrelated self-generated spontaneous trials, but not controlled trials, correlated with mind-wandering survey measures. Overall, we find evidence for a meaningful distinction between stimulus-independent controlled thought and spontaneous mind-wandering based on thought content.