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Ask a Neuroscientist: Why is thinking hard so hard?

by Nick Weiler

Graduate students take questions from the public and answer them on the blog Neuwrite West as part of their Ask the Expert series.



Jason asks: What makes certain mental tasks be perceived as more demanding than others?  For physical tasks, it is pretty ease to see how, say, lifting a 10 lbs barbell would be perceived as easier than lifting one that’s 20 lbs. But why is watching a 1 hour video on, say, physics perceived as more demanding than watching an hour of “Desperate Housewives”?



This is a great question, Jason, and one psychologists and neuroscientists have wondered about for many years. Why is it that we feel mentally exhausted after studying for a test or preparing for a meeting, but we read books or watch movies to relax? All of these activities require your brain, after all! And why is it harder to resist eating a cookie when you've been doing brain work for hours?  

Mental Effort and Executive Control

As brain users, we generally feel as if there is some substance called mental effort, which we all have in limited quantities. We have to budget it carefully because some mental tasks require more of it than others, and if we run out we simply have to wait for it to replenish itself before we can use it again.

A classic and simple test of mental effort is called the Stroop task, which I've included here so you can try it yourself. Try naming aloud the colors of the ovals on the left. If you're not colorblind (sorry!), it should be quite easy. Likewise, reading aloud the words on the right should feel like a "no-brainer" (even though it isn't, really). But try listing the colors those words are printed in and you'll find it's quite a bit harder. Why is that?




The fundamental difficulty you experienced while attempting to name the text colors was that one part of your brain was automatically going ahead and processing the meanings of the words, while another part was processing the color of the text, and you had to decide which one to say.

To accomplish the task, you needed to call on brain regions responsible for executive control – in particular, the anterior cingulate cortex and dorsolateral prefrontal cortex, shown below (Macdonald et al. 2000; Milham, et al. 2003). These executive brain regions allow you to remember the rule I gave you ("say the color, not the word"), then detect and resolve the conflicting possible responses offered by the brain regions responsible for reading words and naming colors. When presented with words, our natural response is to read them, not to name their colors, so the executive regions have to work extra hard when the rule of the game requires you to suppress reading in favor of color naming.

Figure 2: Executive control regions crucial for stroop task. Left panel: Illustration of location of left hemisphere dorsolateral prefrontal cortex.

Right panel: MRI image revealing the mid-line of the human brain with the right hemisphere anterior cingulate cortex highlighted.


So, what does this tell us about mental effort? It suggests that 1) mental tasks require more mental effort when they require more work on the part of executive brain regions to focus our attention on the task at hand and suppress distractors, and 2) some mental tasks (reading words) may be more automatic than others (saying the colors of words), making them easier to focus on when you want to, but harder to suppress when you don't.

These two factors both contribute to understanding the example you gave, Jason, of why studying physics seems harder than watching a soap opera:

First, the entertainment industry is expert in techniques to captivate your attention, while physics lecturers (if they're not Sagan or Feynman) are often not so captivating, leaving it up to your own executive mental discipline to maintain focus on the physics and not, well, everything else that's a shard more interesting.

Second, we are social creatures, wired by evolution and experience to be fascinated by people and their social dynamics. Just like your native language seems "easy" compared to a foreign one, we are socialized to enjoy life spun into story-form from a very young age. This becomes automatic and effortless. Dissecting the math and physics behind the universe is a much more advanced and unfamiliar skill, and therefore takes more work.

But saying that tasks that require a lot of executive control feel "hard" or tasks that are more automatic feel “easy” leaves out the harder (and more interesting) question of why. What about exerting these executive faculties is actually difficult? Why does mental effort feel like a limited resource that you have to replenish when you work your brain too hard?

Fuel vs. Treat

Psychologists and neuroscientists have proposed two main theories for why focus and self-control are difficult to maintain: the resource model and the process model.

The resource model proposes that the brain is like a muscle in that it has some limited resource of mental effort that can get used up by exerting self-control over a long period of time. When this happens, your brain has no choice but to shut down the executive functions until it accumulates more of this mental fuel. This theory was popularized in the late nineties by Roy Baumeister and his notion of "ego depletion". (Baumeister et al 1998), and has been supported by a number of experiments showing a relationship between blood glucose (the brain's main fuel) and self control (Gaillot et al. 2007; Gaillot & Baumeister, 2007; Gaillot, 2008).

The process model (aka the motivation model), proposes that your brain is like a dog who will only do tricks as long as she is relatively confident that she will eventually receive a treat. If it starts looking unlikely that any reward is going to be offered, your brain may lose motivation for the current task, and be more and more inclined to quit barking up the wrong tree, so to speak, and look for easier rewards elsewhere (Kurzban, 2010; Hagger et al, 2010; Inzlicht & Schmeichael, 2012).

One reason it is difficult for scientists to figure out which of these theories is right is that the fundamental fuel and the fundamental reward of the brain involve the same molecule: glucose. As with the rest of your organs, the brain's energy comes from a controlled burn of glucose and oxygen, which circulate in the blood to get to fuel-hungry cells. And while there are many other types of reward recognized by the brain, glucose is one of the most potent.

So, how can I get my brain to work harder?

Either way, it seems clear that there is some cost to executive function, even if scientists have not yet come to a consensus as to whether that cost is immediate (running low on fuel) or future (missing out on possible rewards). This means that mental work is an investment

For the moment, it's probably best to hedge your bets. Your cognitive work will benefit from eating healthily to maintain the glucosic fuel your executive functions need, from taking breaks to let the groundwater of willpower replenish itself, and from giving yourself treats and rewards along the way to encourage your brain to maintain proper motivation.