Colorful visualls, all-connectedness or ego loss. The altered perceptions after taking psychedelic mushrooms are hard to put into words. But what is the reason? What is happening in the brain?

In our brain, information is exchanged through an extensive network of nerve cells (neurons) that form numerous connections and link different areas of the brain. While these are still flexible and changeable in children, the exchange of information in adulthood is limited to a few strongly developed connections. In a conscious state, our thinking takes place via the so-called Default Mode Network (DMN), i.e. the standard connections in only a few brain areas. 

However, the whole thing looks different under the influence of psilocybin. Carhart-Harris et al. 2012 examined subjects by means of MRI and were able to show, as the scans, that after administration of the drug not only the blood flow but also the neuronal activity in the brain areas of the DMN were strongly reduced. The standard connections were thus as if switched off.[1]

Neurons were thus no longer able to fire along their entrenched pathways and had to seek out new connections, as they did in childhood. As a result, a wide variety of brain areas were now more strongly interconnected. In addition, it could be shown that there was considerably greater neuronal activity (represented in the diagram by the different colored areas) [2,3].

Through this restructuring of communication, it is assumed that, on the one hand, rigid thoughts and behavioral patterns of the DMN, which play a role in clinical depression, addiction or obsessive-compulsive disorders, can be broken through again. Furthermore, the linking of different areas can explain altered perceptions, such as the "hearing" of colors.

And this is only the beginning of the clarification of the mode of action of psilocybin!

Sources:

[1] Carhart-Harris, Robin L., et al. "Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin." Proceedings of the National Academy of Sciences 109.6 (2012): 2138-2143.

[2] Carhart-Harris, Robin Lester, et al. "The entropic brain: a theory of conscious states informed by neuroimaging research with psychedelic drugs." Frontiers in human neuroscience 8 (2014): 20.

[3] Petri, Giovanni, et al. "Homological scaffolds of brain functional networks." 

Journal of The Royal Society Interface 11.101 (2014): 20140873.