The Default Mode Network and What Psychedelics Do to It

Over the last two decades, advances in functional neuroimaging have allowed researchers to observe the living brain in real-time, leading to a surge of interest in how classic psychedelics affect neural activity. At the centre of this research is a concept known as the default mode network (DMN). Understanding the DMN has become fundamental to modern theories of consciousness and offers a compelling neurobiological framework for explaining why substances like psilocybin can profoundly alter our sense of self, time, and perception.

What Is the Default Mode Network?

The default mode network is not a single part of the brain, but a group of interacting brain regions—most notably the medial prefrontal cortex and the posterior cingulate cortex—that show highly coordinated activity when a person is awake but not focused on the outside world. It is most active during periods of daydreaming, mind-wandering, and self-reflection. Researchers generally associate the DMN with our autobiographical memory, our ability to think about the past and plan the future, and our construction of a cohesive sense of self, or ego. When we engage in a demanding, externally focused task, the DMN typically deactivates, allowing other brain networks responsible for attention and sensory processing to take over. In essence, it acts as a neurological anchor for our everyday identity and internal narrative.

Disruption of the 'Ego' Under Psychedelics

Classic psychedelics like psilocybin primarily act as agonists at the serotonin 2A (5-HT2A) receptors in the brain. Because these receptors are densely concentrated in the regions that make up the DMN, neuroimaging studies have investigated how psychedelics interact with this network. Using functional magnetic resonance imaging (fMRI), researchers at institutions such as Imperial College London and Johns Hopkins University have consistently observed that psychedelics lead to a marked decrease in blood flow and functional connectivity within the DMN. This disruption is frequently correlated with subjective reports of ego dissolution—the temporary loss of one’s sense of self or feeling at one with the universe—a hallmark of the psychedelic experience. According to leading researchers, the integrity of the DMN appears to depend on normal 5-HT2A signalling, which is disrupted by the presence of these compounds.

Hyperconnectivity and the Entropic Brain

While activity within the DMN decreases under the influence of psychedelics, communication across the rest of the brain appears to increase. Neuroimaging suggests that psychedelics cause a state of hyperconnectivity, where brain regions that do not normally communicate with one another begin to exchange information. This phenomenon is often explained through the 'REBUS' model (RElaxed Beliefs Under pSychedelics), which proposes that psychedelics relax the brain's deeply held priors—its standard, rigid models of how the world works and how the self operates. By temporarily quieting the overriding control of the DMN, the brain enters a state of higher entropy or disorder. In practical terms, this increased brain flexibility is thought to break rigid, maladaptive thought patterns, which is why this mechanism is being heavily researched as a potential adjunct to psychotherapy for treatment-resistant depression and anxiety. Instead of the brain operating on its usual strict, compartmentalised pathways, it temporarily establishes new, freewheeling connections.

Navigating Uncertainty and Legal Contexts

Despite the excitement surrounding these findings, the neuroscience of psychedelics requires careful interpretation. The DMN is a statistical pattern observed in fMRI scans, which measures changes in blood flow as an indirect proxy for neural activity, not direct electrical firing. Furthermore, decreased activity in the DMN does not mean the brain is 'switched off'; rather, it is reorganising how its component parts communicate. The brain is incredibly complex, and the exact mechanisms by which temporary changes in brain connectivity lead to lasting psychological changes remain an area of ongoing study and scientific debate. In South Africa, psilocybin is classified as a Schedule 7 substance under the Medicines and Related Substances Act, meaning that clinical neuroimaging research of this kind is heavily restricted locally, despite the global precedent set by the country's cannabis privacy judgment, which opened the door to drug policy reform conversations. Navigating this legal framework is a necessary step if South African researchers are to eventually contribute to this evolving global neuroscience.