Default Mode Network

Monday, February 23rd, 2015

When Carhart-Harris first began studying the brains of volunteers on psychedelics, neuroscientists assumed that the drugs somehow excited brain activity, which then caused the vivid hallucinations and powerful emotions:

But when Carhart-Harris looked at the results of the first set of fMRI scans — which pinpoint areas of brain activity by mapping local blood flow and oxygen consumption — he discovered that the drug appeared to substantially reduce brain activity in one particular region: the “default-mode network.”

The default-mode network was first described in 2001, in a landmark paper by Marcus Raichle, a neurologist at Washington University, in St. Louis, and it has since become the focus of much discussion in neuroscience. The network comprises a critical and centrally situated hub of brain activity that links parts of the cerebral cortex to deeper, older structures in the brain, such as the limbic system and the hippocampus.

The network, which consumes a significant portion of the brain’s energy, appears to be most active when we are least engaged in attending to the world or to a task. It lights up when we are daydreaming, removed from sensory processing, and engaging in higher-level “meta-cognitive” processes such as self-reflection, mental time travel, rumination, and “theory of mind” — the ability to attribute mental states to others. Carhart-Harris describes the default-mode network variously as the brain’s “orchestra conductor” or “corporate executive” or “capital city,” charged with managing and “holding the entire system together.” It is thought to be the physical counterpart of the autobiographical self, or ego.

“The brain is a hierarchical system,” Carhart-Harris said. “The highest-level parts” — such as the default-mode network — “have an inhibitory influence on the lower-level parts, like emotion and memory.” He discovered that blood flow and electrical activity in the default-mode network dropped off precipitously under the influence of psychedelics, a finding that may help to explain the loss of the sense of self that volunteers reported. (The biggest dropoffs in default-mode-network activity correlated with volunteers’ reports of ego dissolution.) Just before Carhart-Harris published his results, in a 2012 paper in Proceedings of the National Academy of Sciences, a researcher at Yale named Judson Brewer, who was using fMRI to study the brains of experienced meditators, noticed that their default-mode networks had also been quieted relative to those of novice meditators. It appears that, with the ego temporarily out of commission, the boundaries between self and world, subject and object, all dissolve. These are hallmarks of the mystical experience.

If the default-mode network functions as the conductor of the symphony of brain activity, we might expect its temporary disappearance from the stage to lead to an increase in dissonance and mental disorder — as appears to happen during the psychedelic journey. Carhart-Harris has found evidence in scans of brain waves that, when the default-mode network shuts down, other brain regions “are let off the leash.” Mental contents hidden from view (or suppressed) during normal waking consciousness come to the fore: emotions, memories, wishes and fears. Regions that don’t ordinarily communicate directly with one another strike up conversations (neuroscientists sometimes call this “crosstalk”), often with bizarre results. Carhart-Harris thinks that hallucinations occur when the visual-processing centers of the brain, left to their own devices, become more susceptible to the influence of our beliefs and emotions.

Carhart-Harris doesn’t romanticize psychedelics, and he has little patience for the sort of “magical thinking” and “metaphysics” they promote. In his view, the forms of consciousness that psychedelics unleash are regressions to a more “primitive style of cognition.” Following Freud, he says that the mystical experience — whatever its source — returns us to the psychological condition of the infant, who has yet to develop a sense of himself as a bounded individual. The pinnacle of human development is the achievement of the ego, which imposes order on the anarchy of a primitive mind buffeted by magical thinking. (The developmental psychologist Alison Gopnik has speculated that the way young children perceive the world has much in common with the psychedelic experience. As she puts it, “They’re basically tripping all the time.”) The psychoanalytic value of psychedelics, in his view, is that they allow us to bring the workings of the unconscious mind “into an observable space.”


  1. Wangweilin says:

    The assertion of ego in a cooperative dynamic is the basis of human civilization, thus making ego necessary for civilization as we know it.

Leave a Reply