# Neurophysiological Mechanisms of Altered States of Consciousness

### **Default Mode Network (DMN)**

The **Default Mode Network** acts as the brain’s “self-system,” responsible for self-referential thinking, autobiographical memory, and our continuous sense of identity.\
During ASCs, the DMN’s activity changes markedly:

* **Psychedelics** tend to *decrease connectivity* within the DMN, producing experiences of *ego-dissolution* and expanded awareness.
* **Meditation** and **flow states** modulate communication between the DMN and executive networks, promoting *focused attention* and *reduced internal chatter.*

These changes suggest that altering DMN dynamics can temporarily loosen rigid self-models, opening access to new modes of consciousness and creativity.

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### **Entropic Brain Hypothesis (EBH)**

The **Entropic Brain Hypothesis** describes consciousness as existing along a spectrum of *entropy*—a measure of the brain’s informational richness and flexibility.

* **High-entropy states** (psychedelics, REM sleep, early psychosis) are fluid, expansive, and unconstrained.
* **Low-entropy states** (ordinary waking consciousness, depression, anesthesia) are more structured but rigid.

Clinically, psychedelics may *increase neural entropy*, disrupting maladaptive thought patterns and restoring cognitive flexibility—an effect linked to improved mental health outcomes.

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### **Transient Hypofrontality**

This model explains the **temporary downregulation of the prefrontal cortex**—the brain region associated with planning, self-monitoring, and time awareness—during certain altered states.\
When this region quiets down, individuals often experience:

* A *loss of self-consciousness*
* *Distorted sense of time*
* *Effortless performance and absorption*

This mechanism underlies flow, deep meditation, and creative improvisation, highlighting how less control can sometimes unlock more presence.

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### **REBUS Model (Relaxed Beliefs Under Psychedelics)**

The **REBUS model** proposes that psychedelics **soften the brain’s rigid predictive models**, allowing new sensory information to update deeply ingrained beliefs.\
This relaxed state of top-down control enhances:

* *Learning and cognitive flexibility*
* *Emotional release and insight*
* *Integration of new adaptive patterns*

In essence, REBUS describes how psychedelics facilitate a “reset” of the mind’s predictive architecture, enabling transformative psychological change.

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### **Neuroplasticity and Critical Periods**

Emerging evidence suggests that psychedelics and certain ASCs can **reopen windows of heightened neuroplasticity**, similar to those seen during early brain development.\
These temporary “critical periods” allow for:

* *Accelerated neural rewiring*
* *Enhanced emotional learning*
* *Increased capacity for behavioral change*

This mechanism may explain why profound experiences—when properly integrated—can lead to lasting therapeutic and developmental outcomes.<sup>1-4</sup>

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### References

1. Carhart-Harris, R. L.; Leech, R.; Hellyer, P. J.; Shanahan, M.; Feilding, A.; Tagliazucchi, E.; Chialvo, D. R.; Nutt, D. The entropic brain: a theory of conscious states informed by neuroimaging research with psychedelic drugs. *Front. Hum. Neurosci.* 2014, *8*, 20. DOI: 10.3389/fnhum.2014.00020  From NLM PubMed-not-MEDLINE.
2. Herzog, R.; Mediano, P. A. M.; Rosas, F. E.; Lodder, P.; Carhart-Harris, R.; Perl, Y. S.; Tagliazucchi, E.; Cofre, R. A whole-brain model of the neural entropy increase elicited by psychedelic drugs. *Sci. Rep.* 2023, *13* (1), 6244. DOI: 10.1038/s41598-023-32649-7  From NLM Medline.
3. Carhart-Harris, R. L.; Goodwin, G. M. The Therapeutic Potential of Psychedelic Drugs: Past, Present, and Future. *Neuropsychopharmacology* 2017, *42* (11), 2105-2113. DOI: 10.1038/npp.2017.84  From NLM Medline.
4. Greco, A.; Gallitto, G.; D'Alessandro, M.; Rastelli, C. Increased Entropic Brain Dynamics during DeepDream-Induced Altered Perceptual Phenomenology. *Entropy (Basel)* 2021, *23* (7). DOI: 10.3390/e23070839  From NLM PubMed-not-MEDLINE.