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WHAT HAPPENS WHEN, under the influence of psychedelics, the usually firm handshake between brain and world breaks down? No one thing, as it turns out. I asked Carhart-Harris whether the tripping brain favors top-down predictions or bottom-up sensory data. “That’s the classic dilemma,” he suggested: whether the mind, unconstrained, will tend to favor its priors or the evidence of its senses. “You do often find a kind of impetuousness or overzealousness on the part of the priors, as when you see faces in the clouds.” Eager to make sense of the data rushing in, the brain leaps to erroneous conclusions and, sometimes, a hallucination results. (The paranoid does much the same thing, ferociously imposing a false narrative on the stream of incoming information.) But in other cases, the reducing valve opens wide to admit lots more information, unedited and sometimes welcome.

People who are color-blind report being able to see certain colors for the first time when on psychedelics, and there is research to suggest that people hear music differently under the influence of these drugs. They process the timbre, or coloration, of music more acutely—a dimension of music that conveys emotion. When I listened to Bach’s cello suite during my psilocybin journey, I was certain I heard more of it than I ever had, registering shadings and nuances and tones that I hadn’t been able to hear before and haven’t heard since.

Carhart-Harris thinks that psychedelics render the brain’s usual handshake of perception less stable and more slippery. The tripping brain may “slip back and forth” between imposing its priors and admitting the raw evidence of its senses. He suspects that there are moments during the psychedelic experience when confidence in our usual top-down concepts of reality collapses, opening the way for more bottom-up information to get through the filter. But when all that sensory information threatens to overwhelm us, the mind furiously generates new concepts (crazy or brilliant, it hardly matters) to make sense of it all—“and so you might see faces coming out of the rain.

“That’s the brain doing what the brain does”—that is, working to reduce uncertainty by, in effect, telling itself stories.

• • • THE HUMAN BRAIN is an inconceivably complex system—perhaps the most complex system ever to exist—in which an order has emerged, the highest expression of which is the sovereign self and our normal waking consciousness. By adulthood, the brain has gotten very good at observing and testing reality and developing reliable predictions about it that optimize our investments of energy (mental and otherwise) and therefore our chances of survival. Uncertainty is a complex brain’s biggest challenge, and predictive coding evolved to help us reduce it. In general, the kind of precooked or conventionalized thinking this adaptation produces serves us well. But only up to a point.

Precisely where that point lies is a question Robin Carhart-Harris and his colleagues have explored in an ambitious and provocative paper titled “The Entropic Brain: A Theory of Conscious States Informed by Neuroimaging Research with Psychedelic Drugs,” published in Frontiers in Human Neuroscience in 2014. Here, Carhart-Harris attempts to lay out his grand synthesis of psychoanalysis and cognitive brain science. The question at its heart is, do we pay a price for the achievement of order and selfhood in the adult human mind? The paper concludes that we do. While suppressing entropy (in this context, a synonym for uncertainty) in the brain “serves to promote realism, foresight, careful reflection and an ability to recognize and overcome wishful and paranoid fantasies,” at the same time this achievement tends to “constrain cognition” and exert “a limiting or narrowing influence on consciousness.”

After a series of Skype interviews, Robin Carhart-Harris and I were meeting for the first time, in his fifth-floor walk-up in an unposh section of Notting Hill, a few months after the publication of the entropy paper. In person, I was struck by Robin’s youthfulness and intensity. For all his ambition, his affect is strikingly self-effacing and does little to prepare you for his willingness to venture out onto intellectual limbs that would scare off less intrepid scientists.

The entropy paper asks us to conceive of the mind as an uncertainty-reducing machine with a few serious bugs in it. The sheer complexity of the human brain and the greater number of different mental states in its repertoire (as compared with other animals) make the maintenance of order a top priority, lest the system descend into chaos.

Once upon a time, Carhart-Harris writes, the human or protohuman brain exhibited a much more anarchic form of “primary consciousness,” characterized by “magical thinking”—beliefs about the world that have been shaped by wishes and fears and supernatural interpretation. (In primary consciousness, Carhart-Harris writes, “cognition is less meticulous in its sampling of the external world and is instead easily biased by emotion, e.g., wishes and anxieties.”) Magical thinking is one way for human minds to reduce their uncertainty about the world, but it is less than optimal for the success of the species.

A better way to suppress uncertainty and entropy in the human brain emerged with the evolution of the default mode network, Carhart-Harris contends, a brain-regulating system that is absent or undeveloped in lower animals and young children. Along with the default mode network, “a coherent sense of self or ‘ego’ emerges” and, with that, the human capacity for self-reflection and reason. Magical thinking gives way to “a more reality-bound style of thinking, governed by the ego.” Borrowing from Freud, he calls this more highly evolved mode of cognition “secondary consciousness.” Secondary consciousness “pays deference to reality and diligently seeks to represent the world as precisely as possible” in order to minimize “surprise and uncertainty (i.e. entropy).”

The article offers an intriguing graphic depicting a “spectrum of cognitive states,” ranging from high-entropy mental states to low ones. At the high-entropy end of the spectrum, he lists psychedelic states; infant consciousness; early psychosis; magical thinking; and divergent or creative thinking. At the low-entropy end of the spectrum, he lists narrow or rigid thinking; addiction; obsessive-compulsive disorder; depression; anesthesia; and, finally, coma.

Carhart-Harris suggests that the psychological “disorders” at the low-entropy end of the spectrum are not the result of a lack of order in the brain but rather stem from an excess of order. When the grooves of self-reflective thinking deepen and harden, the ego becomes overbearing. This is perhaps most clearly evident in depression, when the ego turns on itself and uncontrollable introspection gradually shades out reality. Carhart-Harris cites research indicating that this debilitating state of mind (sometimes called heavy self-consciousness or depressive realism) may be the result of a hyperactive default mode network, which can trap us in repetitive and destructive loops of rumination that eventually close us off from the world outside. Huxley’s reducing valve contracts to zero. Carhart-Harris believes that people suffering from a whole range of disorders characterized by excessively rigid patterns of thought—including addiction, obsessions, and eating disorders as well as depression—stand to benefit from “the ability of psychedelics to disrupt stereotyped patterns of thought and behavior by disintegrating the patterns of [neural] activity upon which they rest.”

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