The neural geometry of naturalistic surprise: Graded boundary breakthroughs in the human brain

How whole-brain neural states reorganize when events violate observers' predictions remains unclear. Using naturalistic magic-viewing fMRI, we show that more surprising events carry the whole-brain neural state progressively farther beyond the range occupied during non-surprise viewing. This graded departure is accompanied by coordinated reconfiguration across sensory and association systems rather than uniform response amplification. We call this phenomenon a boundary breakthrough. To quantify its depth, we used non-surprise brain states to define a reference range along the surprise-related transition direction. Boundary exceedance (E_BB) expresses the depth beyond this range in bits; each additional bit means that a reference state is half as likely to reach at least that far. On this scale, depth increased with independently rated surprise intensity, specifically in the breakthrough state and not in the paired reference state. Peri-event trajectories showed that the event deepened an excursion already under way. Exploratory analyses linked exceedance to one-week recognition memory in a surprise-dependent manner. Together, these findings identify a graded systems-level relationship between surprise intensity and the departure of the whole-brain neural state from its non-surprise reference range.