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Mendel Kaelen, a Dutch postdoc in the Imperial lab, proposes a more extended snow metaphor: “Think of the brain as a hill covered in snow, and thoughts as sleds gliding down that hill. As one sled after another goes down the hill, a small number of main trails will appear in the snow. And every time a new sled goes down, it will be drawn into the preexisting trails, almost like a magnet.” Those main trails represent the most well-traveled neural connections in your brain, many of them passing through the default mode network. “In time, it becomes more and more difficult to glide down the hill on any other path or in a different direction.

“Think of psychedelics as temporarily flattening the snow. The deeply worn trails disappear, and suddenly the sled can go in other directions, exploring new landscapes and, literally, creating new pathways.” When the snow is freshest, the mind is most impressionable, and the slightest nudge—whether from a song or an intention or a therapist’s suggestion—can powerfully influence its future course.

Robin Carhart-Harris’s theory of the entropic brain represents a promising elaboration on this general idea, and a first stab at a unified theory of mental illness that helps explain all three of the disorders we’ve examined in these pages. A happy brain is a supple and flexible brain, he believes; depression, anxiety, obsession, and the cravings of addiction are how it feels to have a brain that has become excessively rigid or fixed in its pathways and linkages—a brain with more order than is good for it. On the spectrum he lays out (in his entropic brain article) ranging from excessive order to excessive entropy, depression, addiction, and disorders of obsession all fall on the too-much-order end. (Psychosis is on the entropy end of the spectrum, which is why it probably doesn’t respond to psychedelic therapy.)

The therapeutic value of psychedelics, in Carhart-Harris’s view, lies in their ability to temporarily elevate entropy in the inflexible brain, jolting the system out of its default patterns. Carhart-Harris uses the metaphor of annealing from metallurgy: psychedelics introduce energy into the system, giving it the flexibility necessary for it to bend and so change. The Hopkins researchers use a similar metaphor to make the same point: psychedelic therapy creates an interval of maximum plasticity in which, with proper guidance, new patterns of thought and behavior can be learned.

All these metaphors for brain activity are just that—metaphors—and not the thing itself. Yet the neuroimaging of tripping brains that’s been done at Imperial College (and that has since been replicated in several other labs using not only psilocybin but also LSD and ayahuasca) has identified measurable changes in the brain that lend credence to these metaphors. In particular, the changes in activity and connectivity in the default mode network on psychedelics suggest it may be possible to link the felt experience of certain types of mental suffering with something observable—and alterable—in the brain. If the default mode network does what neuroscientists think it does, then an intervention that targets that network has the potential to help relieve several forms of mental illness, including the handful of disorders psychedelic researchers have trialed so far.

So many of the volunteers I spoke to, whether among the dying, the addicted, or the depressed, described feeling mentally “stuck,” captured in ruminative loops they felt powerless to break. They talked about “prisons of the self,” spirals of obsessive introspection that wall them off from other people, nature, their earlier selves, and the present moment. All these thoughts and feelings may be the products of an overactive default mode network, that tightly linked set of brain structures implicated in rumination, self-referential thought, and metacognition—thinking about thinking. It stands to reason that by quieting the brain network responsible for thinking about ourselves, and thinking about thinking about ourselves, we might be able to jump that track, or erase it from the snow.

The default mode network appears to be the seat not only of the ego, or self, but of the mental faculty of time travel as well. The two are of course closely related: without the ability to remember our past and imagine a future, the notion of a coherent self could hardly be said to exist; we define ourselves with reference to our personal history and future objectives. (As meditators eventually discover, if we can manage to stop thinking about the past or future and sink into the present, the self seems to disappear.) Mental time travel is constantly taking us off the frontier of the present moment. This can be highly adaptive; it allows us to learn from the past and plan for the future. But when time travel turns obsessive, it fosters the backward-looking gaze of depression and the forward pitch of anxiety. Addiction, too, seems to involve uncontrollable time travel. The addict uses his habit to organize time: When was the last hit, and when can I get the next?

To say the default mode network is the seat of the self is not a simple proposition, especially when you consider that the self may not be exactly real. Yet we can say there is a set of mental operations, time travel among them, that are associated with the self. Think of it simply as the locus of this particular set of mental activities, many of which appear to have their home in the structures of the default mode network.

Another type of mental activity that neuroimaging has located in the DMN (and specifically in the posterior cingulate cortex) is the work performed by the so-called autobiographical or experiential self: the mental operation responsible for the narratives that link our first person to the world, and so help define us. “This is who I am.” “I don’t deserve to be loved.” “I’m the kind of person without the willpower to break this addiction.” Getting overly attached to these narratives, taking them as fixed truths about ourselves rather than as stories subject to revision, contributes mightily to addiction, depression, and anxiety. Psychedelic therapy seems to weaken the grip of these narratives, perhaps by temporarily disintegrating the parts of the default mode network where they operate.

And then there is the ego, perhaps the most formidable creation of the default mode network, which strives to defend us from threats both internal and external. When all is working as it should be, the ego keeps the organism on track, helping it to realize its goals and provide for its needs, notably for survival and reproduction. It gets the job done. But it is also fundamentally conservative. “The ego keeps us in our grooves,” as Matt Johnson puts it. For better and, sometimes, for worse. For occasionally the ego can become tyrannical and turn its formidable powers on the rest of us.* Perhaps this is the link between the various forms of mental illness that psychedelic therapy seems to help most: all involve a disordered ego—overbearing, punishing, or misdirected.*

In a college commencement address he delivered three years before his suicide, David Foster Wallace asked his audience to “think of the old cliché about ‘the mind being an excellent servant but a terrible master.’ This, like many clichés, so lame and unexciting on the surface, actually expresses a great and terrible truth,” he said.

“It is not the least bit coincidental that adults who commit suicide with firearms almost always shoot themselves in the head. They shoot the terrible master.”