Introduction
The default mode network (DMN) is arguably the most important concept in modern psychedelic neuroscience. First identified through positron emission tomography (PET) studies by Marcus Raichle and colleagues at Washington University in 2001, the DMN is a set of interconnected brain regions that activate when a person is not focused on the external world -- during daydreaming, self-reflection, rumination, and mental time travel. The discovery that psychedelics profoundly disrupt this network has reshaped our understanding of consciousness, selfhood, and mental illness.
What Is the Default Mode Network?
The DMN comprises several key brain regions: the medial prefrontal cortex (mPFC), the posterior cingulate cortex (PCC), the inferior parietal lobule, the lateral temporal cortex, and the hippocampal formation. These regions show correlated activity during rest and deactivate during externally directed tasks. Raichle's group named it the "default mode" because it represents the brain's baseline state -- the mental activity that occurs when you are simply being yourself.
Functionally, the DMN is associated with self-referential processing, autobiographical memory, theory of mind (thinking about others' mental states), and future planning. It is, in a sense, the neural substrate of the narrative self -- the ongoing internal monologue that constructs our sense of identity.
