"A few hours of mountain climbing turn a villain and a saint into two rather equal creatures. Exhaustion is the shortest way to equality and fraternity.” – Friedrich Nietzsche
(a) The Problem
Exercise is beneficial to mental health. Among other things, it reduces stress, decreases anxiety, alleviates depression and enhances cognitive function. Despite decades of research, though, the mechanisms in the brain that mediate these positive effects on mood and cognition are little understood. How, exactly, does putting one foot in front of the other for a certain amount of time makes you feel, well, peachy? And, how on Earth does it facilitate creative insights? Two theories are customarily invoked – in a knee-jerk fashion, almost – to explain this phenomenon. One claims that exercise increases blood flow to the brain (the more-blood-theory or MBT) and the second points the finger at the endorphins. Despite popular belief to the contrary, this, however, does not explain it. While the MBT is wrong outright, the endorphin theory rests on very sketchy evidence indeed and cannot, at any rate, account for the data.
(b) The Theory
There is much renewed interest in the field in recent years, which focuses mostly on molecular and neurochemical changes that take place during exercise. Such changes, however, cannot explain the short-term effects of exercise on emotion and cognition; that is, why, say, running for a mere 20 minutes raises your self-esteem. For such acute effects, especially on higher mental processes, I formulated the transient hypofrontality theory (THT), which is a quick, physiological mechanism based on changes in neural activity. Briefly, t he central idea behind it is that the brain, in order to drive the bodily motion, is forced to make profound changes to the way it allocates its metabolic resources. This follows from the facts that the brain has a finite energy supply and that bodily motion is an extremely demanding task in computational terms; that is, to be clear, for the brain, not the body. In other words, as the brain sustains, during exercise, the massive and widespread neural activation that run motor units, assimilate sensory inputs, and coordinate autonomic regulation, it must take metabolic resources, given their limited availability, away from neural structures whose functions are not critically needed at the time, which are, according to the THT, areas of the prefrontal cortex and, perhaps, limbic system. Charles Darwin had a hunch about this when he wrote in The Expression of Emotions in Animal and Man: “A man cannot think deeply and exert his utmost muscular force.”
(c) The Twist
The THT is initially quite counterintuitive because it contradicts three widely held but mistaken beliefs. These are: (a) exercise boosts blood supply to the brain and, therefore, oxygen and glucose uptake; (b) bodily motion is taxing for muscles, no doubt, but it is not something that forces the brain into its computational reserves like, say, playing chess or writing a philosophical treatise; and (c) any improvements in mental health – stress relief, mood elevation, anxiolysis, that sort of thing – must surely emerge from some process that activates, or at least reactivates, a neural region that was hitherto not running on all four cylinders; think about it, how can decreasing the activity of a brain area, let alone the prefrontal cortex, our most priced cortical piece of real estate, be good for you, eh? The THT, then, postulates that exercise causes a state of prefrontal hypofunction and that it is exactly this deactivation of prefrontal regions that provides (1) the mental health benefits and (2) the changes to phenomenal consciousness and thinking patterns. I proposed the THT for the first time in a paper in 2003 ( Dietrich, 2003), but I published a more exercise specific account of it in 2006 ( Dietrich, 2006) that incorporated more empirical evidence, including my own ( Dietrich, 2004b; Sparling & Dietrich, 2004). Current studies in my lab use neuroimaging to better understand this brain mechanism. We are also working on a more comprehensive neuroscientific model that combines hypofrontality with other neural mechanisms, such as those underlying arousal.
(d) The Endocannabinoid Theory
I was having a chat one day with my friend and colleague Bill McDaniel in the hallway separating our two offices, and we noted that the changes in mental status reported by people engaged in prolonged exercise bear a close resemblance to the pharmacological properties of marijuana – relaxation, sedation, analgesia, among other things. Given the trouble with the endorphins, we thought the endocannabinoid system might be involved instead. We did a little study and the results seemed to indicate just that ( Sparling et al., 2003). This finding meant that the alterations of subjective experience associated with running may not be an endorphin but a cannabinoid high, a suggestion we made in a paper in 2004 ( Dietrich & McDaniel 2004).