How does the brain's energy consumption depend on mental activity?

Question

What is the impact of mental activity on the energy consumption of the human brain?I am most interested in intellectually demanding tasks (e.g., chess matches, solving a puzzle, taking a difficult exam) versus tasks with a similar posture but less demanding (e.g., reading a newspaper, watching TV). I heard that energy consumption stays remarkably constant regardless of the mental activity (and energy consumption can be explained by elevated heart rate due to stress). However, it seems to contradict techniques such as fMRI, where the change in metabolism is measured (unless the relative change is really small).

Answer 1

Interesting question. Firstly, when talking about energy consumption in the brain it is important to remember that almost all metabolic functions occurring there rely on glucose - the brain does not use fats or proteins to generate energy like our other organs do (although it is capable of using ketone bodies). A high baseline metabolic rate does occur in the brain - it is constantly working (for more information about baseline activity you should look into the “default network” of the brain) and therefore has a high energy demand.

When talking about fMRI it is important to recognise that you are not directly measuring metabolic activity - rather you are measuring the blood flow to certain regions of the brain. The logic here is that areas that are more active then others will have a higher metabolic activity and therefore require a greater supply of blood. These imaging techniques normalise baseline levels and subtract them from the levels that occur upon mental activity (say for example when playing chess as you mentioned). In this sense, even small differences can be detected and analyzed.