How to Change Your Mind

(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