One of the hardest myths of the mind to counter is the myth that we only use 10% of our brain. It sounds like an attempt at motivation – think how awesome we could all be, if we just lived up to our potential! – but the brain is in the “use it or lose it” school of tough love, and if we only used 10% of it, our heads could be a great deal smaller, or emptier.
Although it may not feel like it, even when you think you aren’t using your brain for anything in particular there’s a great deal of activity going on in there. In fact, the brain “at rest” is one of the breakthroughs of 21st century neuroscience, dubbed “the default network”.
Despite assertions that the brain is never truly idle by both Hans Berger (father of the EEG) in the 1920s and David Ingvar (early brain mapper) in the 1970s, many psychologists maintained a persistent belief that the brain did nothing worth mentioning unless people were actively trying to complete a particular task. This led to a number of questionable conclusions about the brain…but also to the discovery of our default network.
You see, neuroimaging research works by comparing brain activity at time X to brain activity at time Y. In the early days of neuroimaging – back when we more likely to inject volunteers with radioactive isotopes than surround them with superstrong magnets – “time Y” was typically just volunteers sitting there with no instructions, minds wandering wherever they were wont to go. You can imagine the frustration and surprise some early neuroscientists must have felt when they saw that the tasks they were interested in increased activity in some regions of the brain, but also decreased activity in others. How could the brain possibly use more activity when people were idle than it did when they were completing whatever challenging, interesting task the researchers had designed?
This question intrigued a group of scientists, led by Gordon Shulman, enough to analyze the data from several different neuroimgaing studies and demonstrate that the same brain regions were being deactivated by a variety of tasks, suggesting those brain regions were responsible for “ongoing processes”. A few years later, this was clarified as the “default mode” of our brains, the shore our minds land on when we allow them to drift.
In other words, when you aren’t trying to think or do anything in particular, your mind is still abuzz with activity. Specifically, it will be abuzz in the default network, whose main players are three specific brain regions:
- The medial temporal lobe, focused on the hippocampus. This region of the brain was made famous by the study of patient H. M., who underwent surgery to remove these regions (to stop terrible seizures) and subsequently could form no new autobiographic memories. Thanks to H. M., we know the medial temporal lobe is a critical component of memory.
- The medial prefrontal cortex, a subsection of that part of the brain immediately behind your forehead that seems critical for higher-level cognition such as regulating our behavior, processing social information, and most critically, thinking about ourselves and other people.
- The posterior cingulate cortex, a less-understood brain region that may help us motivate or recognize a need to change our behavior, perhaps by integrating information from many different brain systems.
A network focused on just three regions of the brain still doesn’t mean that “we only use 10% of our brains when we rest”. These are only the brain regions that are consistently activated in that default mode, for virtually everyone; we probably use other regions depending on exactly where our mind has wandered to, such as visual regions if we’re trying to build a particular image in our head.
Based on those regions that are consistently active when we let our minds roam free, the default network itself seems to be about remembering the past, projecting to the future, and trying to figure out what other people are thinking. Perhaps this isn’t surprising; these are the kinds of spontaneous thoughts our minds seem to gravitate to when we don’t direct them in other directions.
And so, far from being uninteresting and idle, our undirected minds in their default state may be a critical component of consciousness. Or perhaps not consciousness itself, but the continuity of consciousness, as we constantly connect back to past events and project ourselves forward in time, creating the sense that “I” am the same person I was when I was seven, and will be when I’m seventy. Thus the most important work our brains do may be what they get up to when we aren’t telling them to do anything at all – and they need far more than 10% of their neurons to achieve it.
Buckner RL, Andrews-Hanna JR, & Schacter DL (2008). The brain’s default network: anatomy, function, and relevance to disease. Annals of the New York Academy of Sciences, 1124, 1-38 PMID: 18400922