SCIENTIFICAMERICAN.COM | 9organ are more active than others. We have all heard the saying
that people use a small fraction of their brain capacity. In fact,
the entire brain is active at any point in time, but a given task
modulates the activity of only a portion of the brain from its base-
line level of activity.
That arrangement does not mean that you fulfill only half of
your cognitive potential. In fact, if your entire brain were strongly
active at the same time, it would be as if all the orchestra members
were playing as loudly as possible—and that scenario would create
chaos, not enable communication. The deafening sound would not
convey the emotional overtones present in a great musical piece.
It is the pitch, rhythms, tempo and strategic pauses that commu-
nicate information, both during a symphony and inside your head.
MODULARITY
Just as an orchestra can be divided into groups of instru-
ments from different families, the brain can be separated into
collections of nodes called modules—a description of localized
networks. All brains are modular. Even the 302-neuron network
of the nematode Caenorhabditis elegans has a modular struc-
ture. Nodes within a module share stronger connections to one
another than to nodes in other modules.
Each module in the brain has a certain function, just as ev-
ery family of instruments plays a role in the symphony. We re-
cently performed an evaluation of a large number of indepen-
dent studies—a meta-analysis—that included more than 10,
functional magnetic resonance imaging (fMRI) experiments of
subjects performing 83 different cognitive tasks and discovered
that separate tasks map to different brain-network modules.
There are modules occupied with attention, memory and intro-
spective thought. Other modules, we found, are dedicated to
hearing, motor movement and vision.
These sensory and motor cognitive processes involve single,
contiguous modules, most of which are confined to one lobe of
the brain. We also found that computations in modules do not
spur more activity in other modules—a critical aspect of modu-
lar processing. Imagine a scenario in which every musician in
an orchestra had to change the notes played every time anoth-
er musician changed his or her notes. The orchestra would spi-
ral out of control and would certainly not produce aesthetically
pleasing sounds. Processing in the brain is similar—each mod-
ule must be able to function mostly independently. Philosophers
as early as Plato and as recent as Jerry Fodor have noted this ne-
cessity, and our research confirms it.
Even though brain modules are largely independent, a sym-
phony requires that families of instruments be played in unison.
Information generated by one module must eventually be inte-
grated with other modules. Watching a movie with only a brain
module for vision—without access to the one for emotions—
would detract greatly from the experience.
For that reason, to complete many cognitive tasks, modules
must often work together. A short-term memory task—holding a
new phone number in your head—requires the cooperation of au-
ditory, attention and memory-processing modules. To integrate
and control the activity of multiple modules, the brain uses hubs—
nodes where connections from the brain’s different modules meet.
Some key modules that control and integrate brain activity
are less circumspect than others in their doings. Their connec-
tions extend globally to multiple brain lobes. The frontoparie-
tal control module spans the frontal, parietal and temporal lobes.
It developed relatively recently on the timescale of evolution.
The module is especially large in humans, relative to our closest
primate ancestors. It is analogous to an orchestra conductor and
becomes active across a large number of cognitive tasks.
The frontoparietal module ensures that the brain’s multiple
modules function in unison. It is heavily involved in what is
called executive function, which encompasses the separate pro-
cesses of decision-making, short-term memory and cognitive
control. The last is the ability to develop complex strategies and
inhibit inappropriate behavior.
Another highly interconnected module is the salience mod-
ule, which hooks up to the frontoparietal control module and
contributes to a range of behaviors related to attention and re-
sponding to novel stimuli. For example, take a look at two words:
blue and red. If you are asked to respond with the color of the
word, you will react much faster to the one set in red. The fron-
toparietal and salience modules activate when responding to
the color green because you have to suppress a natural inclina-
tion to read the word as “blue.”
Finally, the default mode module spans the same lobes as the
frontoparietal control network. It contains many hubs and is
linked to a variety of cognitive tasks, including introspective
thought, learning, memory retrieval, emotional processing, in-
ference of the mental state of others and even gambling. Criti-
cally, damage to these hub-rich modules disturbs functional con-
nections throughout the brain and causes widespread cognitive
difficulties, just as bad weather at a hub airport delays air traf-
fic all over the country.PERSONAL CONNECTIONS
although our brains have certain basic network compo-
nents—modules interconnected by hubs—each of us shows slight
variations in the way our neural circuits are wired. Researchers
have devoted intense scrutiny to this diversity. In an initial phase
of what is called the Human Connectome Project, 1,200 young
people volunteered to participate in a study of brain-network ar-
chitecture, funded by the National Institutes of Health. (The fi-
nal goal of the project is to cover the entire life span.) Each indi-
vidual’s structural and functional connectivity networks were
probed using fMRI. These data were supplemented by a cogni-
tive battery of testing and questionnaires to analyze 280 behav-
ioral and cognitive traits. Participants provided information about
how well they slept, how often they drank alcohol, their language
and memory abilities, and their emotional states. Neuroscientists
from all over the world have been poring over this incredibly rich
data set to learn how our brain networks encode who we are.
Using data from hundreds of participants in the Human Con-
nectome Project, our lab and others have demonstrated that
brain-connectivity patterns establish a “fingerprint” that distin-
guishes each individual. People with strong functional connec-
tions among certain regions have an extensive vocabulary and
exhibit higher fluid intelligence—helpful for solving novel prob-
lems—and are able to delay gratification. They tend to have more
education and life satisfaction and better memory and attention.
Others with weaker functional connections among those same
brain areas have lower fluid intelligence, histories of substance
use, poor sleep and a decreased capacity for concentration.
Inspired by this research, we showed that the findings could