Bahi, C., Irrmischer, M., et al., 2024.

Effects of conscious connected breathing on cortical brain activity, mood and state of consciousness in healthy adults

Current Psychology vol. 43 no. 12 p. 10578–10589

Uses EEG and psychometric measures (11D-ASC, Profile of Mood States) to measure the effect of conscious connected breathing.

Hirschfeld, T. and Schmidt, T., 2021.

Dose–response relationships of psilocybin-induced subjective experiences in humans

Journal of Psychopharmacology vol. 35 no. 4 p. 384–397

A meta-study measuring the dose-dependent effect of psilocybin on the subjective experiences in humans, as measured by the 11D-ASC.

Havenith, M., Leidenberger, M., et al., 2025.

Decreased CO2 saturation during circular breathwork supports emergence of altered states of consciousness

Communications Psychology vol. 3 no. 1

Correlates the effect of breathwork on the 11D-ASC with CO2 saturation in the breath. While the role of CO2 is well known, this study shows that the concentration of CO2 directly correlates with the intensity of the experience.
Altered States Database

Altered States Database

https://alteredstatesdb.org

A database of dozens of studies that induce and measure altered states of consciousness with various methods (both with psychedelics, and non-chemical methods like breathwork and meditation).