Harris got a surprise: “We were seeing decreases in blood flow”—blood
flow being one of the proxies for brain activity that fMRI measures. “Had
we made a mistake? It was a real head-scratcher.” But the initial data on
blood flow was corroborated by a second measure that looks at changes in
oxygen consumption to pinpoint areas of elevated brain activity. Carhart-
Harris and his colleagues had discovered that psilocybin reduces brain
activity, with the falloff concentrated in one particular brain network that
at the time he knew little about: the default mode network.
Carhart-Harris began reading up on it. The default mode network, or
DMN, was not known to brain science until 2001. That was when Marcus
Raichle, a neurologist at Washington University, described it in a
landmark paper published in the Proceedings of the National Academy
of Sciences, or PNAS. The network forms a critical and centrally located
hub of brain activity that links parts of the cerebral cortex to deeper (and
older) structures involved in memory and emotion.
The discovery of the default mode network was actually a scientific
accident, a happy by-product of the use of brain-imaging technologies in
brain research. The typical fMRI experiment begins by establishing a
“resting state” baseline for neural activity as the volunteer sits quietly in
the scanner awaiting whatever tests the researcher has in store. Raichle
had noticed that several areas in the brain exhibited heightened activity
precisely when his subjects were doing nothing mentally. This was the
brain’s “default mode,” the network of brain structures that light up with
activity when there are no demands on our attention and we have no
mental task to perform. Put another way, Raichle had discovered the
place where our minds go to wander—to daydream, ruminate, travel in
time, reflect on ourselves, and worry. It may be through these very
structures that the stream of our consciousness flows.
The default network stands in a kind of seesaw relationship with the
attentional networks that wake up whenever the outside world demands
our attention; when one is active, the other goes quiet, and vice versa. But
as any person can tell you, quite a lot happens in the mind when nothing
much is going on outside us. (In fact, the DMN consumes a
disproportionate share of the brain’s energy.) Working at a remove from
our sensory processing of the outside world, the default mode is most
active when we are engaged in higher-level “metacognitive” processes
such as self-reflection, mental time travel, mental constructions (such as
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