Autopilot (5 page)
When anatomically distinct regions of the brain collaborate, as during Aunt Lisa's visit, they temporarily form “functional networks.” These networks are functional in the sense that they are only formed in order to accomplish a certain task, such as to store some new factoid from Aunt Lisa. These networks can be short-lived, only lasting a few hundred milliseconds. One unresolved question in neuroscience is whether or not temporary functional networks can alter their underlying structural networks. In other words, if air traffic going to and from Bozeman, Montana were to increase beyond this airport's capacity, would the city expand the airport, which might lead to even more air traffic?
There is evidence of large scale plasticity in musicians who, compared to non-musicians, have much larger neural structures that represent their hands and fingers in the motor cortex. But presumably these changes take place over many years of training. The same is true of bilinguals: they have extra neural structures for languages in the temporal regions of the brain. London cabbies have famously large hippocampuses, specifically in the regions that help us navigate and remember spatial locations. It's as if the brain decided to expand the airports in these areas to allow for the increased demand in traffic. It's unknown how fast this type of structural change can happen in the brain. What we do know now is that brain plasticity is possible throughout our lifespan. So it truly is never too late to learn a new instrument, to learn a new language, or to radically change your life: your brain will change, too.
As an adult, these changes may be more stressful, but they are often good for your brain's long-term health. What's also unknown is whether or not lazy people have larger or more active default mode networks. Would this be a cause or a result of being idle? If ten thousand hours of practice are needed to become an expert violinist, how many hours of being idle are required to become a master idler?
The measure of how well the nodes in your default mode network are communicating is called “functional connectivity.” Functional connectivity is used to indicate how well your default mode network is working, and can provide information about your brain health in general, like the measure of how fast and safely air traffic travels between airports.
When you are at rest, fMRI data can be used to see whether the nodes in your default mode network are active together. It is possible to see if oxygen in the blood at these regions increases or decreases at the same time. If you have a healthy brain and you are at rest, you will have high functional connectivity in your default mode network. As you age, if you don't get enough sleep, if you have Alzheimer's disease, or if you've had a stroke, the functional connectivity in your brain decreases, perhaps because of damage to nodes in the network.
It follows that a lifetime of being super-productive and pointlessly-busy might also decrease the functional connectivity in your default mode network. Until Marcus Raichle discovered the default mode network, the only functional or structural networks neuroscientists thought were important were the ones they studied, which became active during tightly controlled experiments. This is because most brain scientists and psychologists assume that the brain's primary purpose is to process external information.
Until very recently, it has only been possible to study how humans respond to external stimulation. It wasn't until we developed the technology to see inside the living brain and study its activity during idleness that we discovered that most of the brain's activity is dedicated to internal operations.
This does not in any way reduce the importance of what we've learned about how different systems in the brain respond to the environment. The motor system, for example, forms and executes commands to your nerves and muscles in your limbs to carry out actions, or to react to events in the world, such as an incoming tennis serve. This system has been studied for decades. But it turns out that when the motor system engages and tells your arm to swing a tennis racket after (or actually
before
) your visual system has reported an incoming serve, it might be only using a very tiny fraction of your brain's total energy.
While it is vitally important that neuroscience discovers what it can about the motor system, it may only be scratching the surface to study discrete areas of the brain while ignoring the “noise” of the resting brain. Noise, technically speaking, is some unwanted signal that usually interferes randomly with whatever signal we are studying. But the network that Raichle observed seemed to “deactivate” during active concentration on a stimulus and did not behave randomly. Nor did it interfere with signals of interest. It behaved perfectly regularly: when a subject begins actively thinking about something, this network deactivates.
Why would a network in the brain decrease its activity during targeted mental tasks like remembering a list of words? Even more mysterious is the fact that the network decreases its activity regardless of the mental task in question. Looking at many different experimental conditions, the same thing happened: this network deactivated as soon as the subject began to perform an experimental task. Naturally, he wondered what happened to this network when people just lay there doing nothing. It turned out that the brain's noise wasn't “noise” at all.
What Raichle found so striking was that many scientists still doubt that it is possible. They argue that it's a measurement error, some technical problem, or an artifact of how fMRI data is analyzed. When subjects just lie in the MRI scanner and let their minds wander, the exact same network that deactivated during experimental tasks begins to hum with activity.
Additionally, during mind-wandering the activity in the nodes in this network becomes highly correlated. This means that each part of the default mode network behaves the same way. Crucially, the default network that activates during idleness is almost perfectly “anti-correlated”
with the network that activates during tasks that require your attention. You can probably guess what an anti-correlation is: the opposite of correlation. Something “X” which is anti-correlated with “Y” means that when the value of X goes up, the value of Y goes down, and vice versa.
Using fMRI data, the signal that neuroscientists use to measure the activity of a certain brain region is called the Blood-Oxygen-Level-Dependent (BOLD) contrast. Without going into the complicated details, this signal tells you roughly how much blood and oxygen is flowing to an active brain region. When neurons increase their activity, they use more blood and oxygen (just like your muscles). A rise in the BOLD signal indicates an increase in brain activity.
Even though the network your brain uses to actively pay attention only requires a small fraction of your brain's total energy, when this attention network activates, your default mode network reduces its activity. This is what is meant by anti-correlated: when your attention network activates, your default mode deactivates. While you run around like a decapitated chicken in your daily life, trying to manage your schedule, trying to keep up with all your mobile devices, posting to your Twitter and Facebook accounts, receiving text messages, composing emails, and checking off to-do lists, you are suppressing the activity of perhaps the most important network in your brain.
The two networks I have been describing are also referred to as the “task positive network” (TPN) and the “task negative network” (TNN). The task negative network is the same as the default mode network. The task positive network is the one that becomes active when you are frantically trying to manage your time.
What all this means is that as you lie there letting your mind wanderâor in the awkward language of neuroscientific writing, having Stimulus Independent Thoughtsâyour brain becomes
more
organized than if you are trying to concentrate on some task like color coding your Outlook calendar. Thus, when you space out, information begins to flow between the nodes in the default mode network. The activity in these regions and in the network as a whole increases. We shall see later why this might be so crucial to your creative mind, and to your health in general.
Where and what exactly is the default mode network? The default mode network arises from a set of posterior, medial, anterior medial, and lateral parietal brain regions. Posterior means “behind,” medial means “middle,” anterior medial means “middle front,” and lateral parietal means regions that are on both sides toward the top and back of your head. The specific regions that form the default mode network are called: medial prefrontal cortex, the anterior cingulate cortex, the precuneous, the hippocampus, and the lateral parietal cortex.
What is important is to realize that these regions form nodes in the very large widespread network that is your default mode. These nodes are brain hubs. It is as if the default mode network comprises O'Hare, JFK, Heathrow, and Frankfurt airports. Together these nodes form the “epicenter” of your brain's activity.
In the back of your brain (posterior) sits the precuneous. The precuneous is a hidden brain structure because it is close to the division between your brain's hemispheres and parts of it are deep in your brain.
The precuneous has been difficult to study because of its location and because isolated injury to this region is rare. Therefore we cannot study patients who have had a stroke in the precuneous to find out what has been impaired. What we do know is that it is involved in spatial reasoning and consciousness. Interestingly, the precuneous also plays a role in self-processing operations like reflecting and maintaining a first-person perspective. Recent analysis using graph theory also indicates that the precuneous is a hub node, in addition to being part of the default mode network. Like O'Hare or Atlanta's Hartsfield Airport, it has a lot of traffic.
During experimental tasks, or in real life when your attention is directed to a PowerPoint about risk management, the precuneous shows less activity. When you are stressing at work about the slip in the project schedule or doing “deep-dives” to find out why a product failed, this region deactivates. In other words: precuneous just doesn't care.
However, the precuneous is also one of the regions that show the highest resting metabolic rate of any region in the brain. This means that at rest the precuneous starts devouring glucose like a crazed hummingbird. So if you can decouple from your “lean” workplace and start doing nothing, this hub in your default mode network revs up and starts redlining. Why is that important? The precuneous seems to be involved in self-reflection. One of the best ways to get to know yourself is to find a quiet or comfortably noisy place, stare at the sky, space out for a while and see what the precuneous gets up to.
Like the precuneous, the parietal cortex is also involved in representing you to yourself, sometimes called “meta-cognition.” The ability to think about this question and to have some kind of answer comes partly from our lateral parietal cortex. Life would be pretty meaningless if you lacked any awareness of yourself.
Coherent conscious representations of our own selves may be one of the unique traits of human cognition, along with language. Does a frog know he is a frog? Our own identities are of course based on these representations. Crucially, the lateral parietal cortex allows you to know whether you are a goth, a punk, a hipster, or a brain scientist. The lateral parietal cortex is also a node in the default network and thus its activity
decreases
during externally induced mental tasks. Like the precuneous, the lateral parietal cortex is also a hub node.
This may be why as you start daydreaming at work, when you should be tracking the hours spent on the latest rollout schedule for synergizing marketing plans across business units, your thoughts invariably drift toward questions like “how did a vibrant and wonderful person like me end up doing something so stupid, meaningless, soul-crushing, and mind-numbing?” Your default mode network knows you better than anyoneâincluding your “getting things done” self.
The next part of the default mode network, called the anterior cingulate cortex (abbreviated as ACC), requires a short digression. You know already that your brain has two halvesâcalled hemispheres. The hemispheres are connected via a fiber tract called the corpus callosum.
The corpus callosum allows information to flow between the two hemispheres. Sometimes, this fiber tract is cut surgically to prevent seizures in people with intractable epilepsy. Sitting like a collar wrapped around the corpus callosum is the anterior cingulate cortex. It is connected to the prefrontal cortex.
One of the anterior cingulate's primary roles is to monitor your behavior together with feedback from your environment and to let you know when you've made a mistake. This is called “error detection.” In a similar way, when you are idle the anterior cingulate also seems to monitor your subconscious for potential solutions to problems.
When the ACC discovers some remotely associated concepts that might work together in a novel idea it directs your attention to this idea, thus boosting its activation so that the idea can enter your consciousness. As part of the default mode network, the ACC likes it when you are taking it easy and are in a positive mood. During idleness, it appears to be ready to help you find insightful solutions and come up with creative thoughts. When you are stressed out and worried about external concerns, the activity of the ACC decreases.
Journeying to the center of the brain, we find the hippocampus. This is one of the most studied brain regions because it is what allows us to form memories. In fact, there is an academic journal devoted entirely to the study of the hippocampus, unsurprisingly if unimaginatively called
Hippocampus.
The hippocampus is a horseshoe-shaped structure deep in the middle of the brain. It has two halves which straddle the brain's left and right hemispheres. As with all brain regions, the hippocampus seems to have a primary functionâforming memoriesâbut its sub-regions perform specialized tasks that range from learning how to navigate new spaces to creating new autobiographical memories.
