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This sense of merging into some larger totality is of course one of the hallmarks of the mystical experience; our sense of individuality and separateness hinges on a bounded self and a clear demarcation between subject and object. But all that may be a mental construction, a kind of illusion—just as the Buddhists have been trying to tell us. The psychedelic experience of “non-duality” suggests that consciousness survives the disappearance of the self, that it is not so indispensable as we—and it—like to think. Carhart-Harris suspects that the loss of a clear distinction between subject and object might help explain another feature of the mystical experience: the fact that the insights it sponsors are felt to be objectively true—revealed truths rather than plain old insights. It could be that in order to judge an insight as merely subjective, one person’s opinion, you must first have a sense of subjectivity. Which is precisely what the mystic on psychedelics has lost.

The mystical experience may just be what it feels like when you deactivate the brain’s default mode network. This can be achieved any number of ways: through psychedelics and meditation, as Robin Carhart-Harris and Judson Brewer have demonstrated, but perhaps also by means of certain breathing exercises (like holotropic breathwork), sensory deprivation, fasting, prayer, overwhelming experiences of awe, extreme sports, near-death experiences, and so on. What would scans of brains in the midst of those activities reveal? We can only speculate, but quite possibly we would see the same quieting of the default mode network Brewer and Carhart-Harris have found. This quieting might be accomplished by restricting blood flow to the network, or by stimulating the serotonin 2A receptors in the cortex, or by otherwise disturbing the oscillatory rhythms that normally organize the brain. But however it happens, taking this particular network off-line may give us access to extraordinary states of consciousness—moments of oneness or ecstasy that are no less wondrous for having a physical cause.

• • • IF THE DEFAULT MODE network is the conductor of the symphony of brain activity, you would expect its temporary absence from the stage to lead to an increase in dissonance and mental disorder—as indeed appears to happen during the psychedelic journey. In a series of subsequent experiments using a variety of brain-imaging techniques, Carhart-Harris and his colleagues began to study what happens elsewhere in the neural orchestra when the default mode network puts down its baton.

Taken as a whole, the default mode network exerts an inhibitory influence on other parts of the brain, notably including the limbic regions involved in emotion and memory, in much the same way Freud conceived of the ego keeping the anarchic forces of the unconscious id in check. (David Nutt puts the matter bluntly, claiming that in the DMN “we’ve found the neural correlate for repression.”) Carhart-Harris hypothesizes that these and other centers of mental activity are “let off the leash” when the default mode leaves the stage, and in fact brain scans show an increase in activity (as reflected by increases in blood flow and oxygen consumption) in several other brain regions, including the limbic regions, under the influence of psychedelics. This disinhibition might explain why material that is unavailable to us during normal waking consciousness now floats to the surface of our awareness, including emotions and memories and, sometimes, long-buried childhood traumas. It is for this reason that some scientists and psychotherapists believe psychedelics can be profitably used to surface and explore the contents of the unconscious mind.

But the default mode network doesn’t only exert top-down control over material arising from within; it also helps regulate what is let into consciousness from the world outside. It operates as a kind of filter (or “reducing valve”) charged with admitting only that “measly trickle” of information required for us to get through the day. If not for the brain’s filtering mechanisms, the torrent of information the senses make available to our brains at any given moment might prove difficult to process—as indeed is sometimes the case during the psychedelic experience. “The question,” as David Nutt puts it, “is why the brain is ordinarily so constrained rather than so open?” The answer may be as simple as “efficiency.” Today most neuroscientists work under a paradigm of the brain as a prediction-making machine. To form a perception of something out in the world, the brain takes in as little sensory information as it needs to make an educated guess. We are forever cutting to the chase, basically, and leaping to conclusions, relying on prior experience to inform current perception.

The mask experiment I attempted to perform during my psilocybin journey is a powerful demonstration of this phenomenon. At least when it is working normally, the brain, presented with a few visual clues suggesting it is looking at a face, insists on seeing the face as a convex structure even when it is not, because that’s the way faces usually are.

The philosophical implications of “predictive coding” are deep and strange. The model suggests that our perceptions of the world offer us not a literal transcription of reality but rather a seamless illusion woven from both the data of our senses and the models in our memories. Normal waking consciousness feels perfectly transparent, and yet it is less a window on reality than the product of our imaginations—a kind of controlled hallucination. This raises a question: How is normal waking consciousness any different from other, seemingly less faithful productions of our imagination—such as dreams or psychotic delusions or psychedelic trips? In fact, all these states of consciousness are “imagined”: they’re mental constructs that weave together some news of the world with priors of various kinds. But in the case of normal waking consciousness, the handshake between the data of our senses and our preconceptions is especially firm. That’s because it is subject to a continual process of reality testing, as when you reach out to confirm the existence of the object in your visual field or, upon waking from a nightmare, consult your memory to see if you really did show up to teach a class without any clothes on. Unlike these other states of consciousness, ordinary waking consciousness has been optimized by natural selection to best facilitate our everyday survival.

Indeed, that feeling of transparency we associate with ordinary consciousness may owe more to familiarity and habit than it does to verisimilitude. As a psychonaut acquaintance put it to me, “If it were possible to temporarily experience another person’s mental state, my guess is that it would feel more like a psychedelic state than a ‘normal’ state, because of its massive disparity with whatever mental state is habitual with you.”

Another trippy thought experiment is to try to imagine the world as it appears to a creature with an entirely different sensory apparatus and way of life. You quickly realize there is no single reality out there waiting to be faithfully and comprehensively transcribed. Our senses have evolved for a much narrower purpose and take in only what serves our needs as animals of a particular kind. The bee perceives a substantially different spectrum of light than we do; to look at the world through its eyes is to perceive ultraviolet markings on the petals of flowers (evolved to guide their landings like runway lights) that don’t exist for us. That example is at least a kind of seeing—a sense we happen to share with bees. But how do we even begin to conceive of the sense that allows bees to register (through the hairs on their legs) the electromagnetic fields that plants produce? (A weak charge indicates another bee has recently visited the flower; depleted of nectar, it’s probably not worth a stop.) Then there is the world according to an octopus! Imagine how differently reality presents itself to a brain that has been so radically decentralized, its intelligence distributed across eight arms so that each of them can taste, touch, and even make its own “decisions” without consulting headquarters.

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