A single-session fMRI study in which participants breathed with a visual pacer whose rate increased, decreased, or held constant on each trial (with abrupt vs gradual change salience), interleaved with free-breathing periods, and reported the perceived direction of change with confidence and arousal ratings. Respiration was recorded with a chest belt and expired CO2 with a capnograph. This registration covers the functional imaging analysis only: all behavioural and physiological processing is complete and disclosed; no functional image statistic has been computed. Hypotheses include direct replications of Farb, Zuo & Price (2023, eNeuro): insula/opercular deactivation under exogenous respiratory pacing, and MAIA-moderated sparing of anterior cingulate activity.