Two rather expensive methods are functional imaging of the brain using Positron Emission Tomography (PET) or Functional Magnetic Resonance Imaging (fMRI).

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Hi Ziad, thank you so much for your response. I guess this begs the question - why not use a cheaper method (i.e. respiratory gas analysis via respirometry)? Respirometry wouldn't pick where in the brain the activity is happening, granted, but would give a measure of increase in body metabolic rate, i.e. the energy cost.

I agree with you Lewis, but I think that we are talking about minute energy that may not be detected via respirometry. If you have the device it is worthwhile designing a pilot experiment.

Kind regards!

Good question. It would be relatively easy to perform a study with a spirometry device. What cognitive actvity would you propose that would be "thinking hard"? How would you quantitively define thinking hard? This could mean different things to different people. Once this is defined, you can measure for instance the R/Q quotient via spirometry.

There are some studies on the topic. This is a quite controversial matter. Since the brain only uses glucose, scientists monitore glycemia most of the time to see if there is an increase of brain work when people think harder than usual. Some results indicate a decrease of glycemia while other results don't.

Thank you Michael and Neil. An interesting issue could be whether what we perceive as 'thinking hard' actually requires more energy consumption by the brain than when not perceptibly thinking hard; of course it could be that when not 'thinking hard' the brain is nonetheless overall just as active. Anyway, assuming that at least the majority of glucose use by the brain is via aerobic metabolism, a respirometry system ought to pick up any changes in brain energy expenditure or, arguably, at least any changes of a size that are ecologicall relevant.

So, what you are hypothesizing is that while "thinking hard", (which clearly needs to be determined what activity this would entail, and for how long), one would find an increase in CO2, or the RQ quotient would move more closer to 1.0 instead of 0.8. Also, are you looking to do this during MRT/PET scan, or just with spirometry? Measuring arterial blood gases might also yield some clues.

If was to say, monitor someone while they were learning a new language, such as a Rosetta Stone program, for 30 minutes, would that define "hard thinking"?

I don't have access to MRT or PET scans, so to do such a study without gaining some research funding, I'd be focussing on re(spirometry). Possibly blood gas analysis too. My hope is that respirometry is not only cheaper but also arguably better for answering the question - does hard thinking increase overall metabolic rate? I.e. if we wanted to know if physical activity increased metabolic rate, we would plug the participant into a mask rather than a scanner, so why not do the same for mental activity? Further, and out of interest, I wonder if simply measuring increases in rate of CO2 production and/or rate of oxygen consumption might be best for the spirometry, because with RQ being a ratio this measure tends to have more noise. Assuming the brain reaches a 'physiological steady state' within a few minutes, similarly to the way that the body does during constant exercise, perhaps participants need only be subjected to the 'hard thinking' for a few minutes. Good idea about the language learning - I'm a beginner in learning Hungarian and certainly progressing with this language requires a lot of effort on my part!

Also, every subject would have to either lay completely still supine, or in a semi-fowlers position. You would want to eliminate any and all skeletal muscle activity, as to reduce CO2 output. A spirometry could measure RQ, V02, Exp. C02, etc, so it would be a good choice for non-invasive equipment. Perhaps also, core body temperature, via non-invasive double sensor on the forehead to measure heat flux. Quite an interesting topic which could be easily accomplished. If you are into a cooperative effort, let me know.

Dear Lewis - It seems that you are set for a new endeavor. I look forward to seeing the results that you will get from this rather interesting idea.

Good Luck and Kind Regards!

Look forward to the indirect calorimetry study. Pilot data from my lab show no detectable increase in energy expenditure during 30 min incongruent Stroop task, a demanding cognitive task. With regards to blood glucose, we could not find any difference between hard and easy cognitive activity even when prolonged for 90 min (http://jap.physiology.org/content/106/3/857). We replicated and published this finding few times already

Hi Samuele, thank you very much for your input. To clarify, are you saying that you've run indirect calorimetry-based studies to see if the stroop task increased rate of oxygen consumption, and with no discernible effect? Are you intending to publish your results? If so then great (I hope the lack of significant results won't stop you) and I think you have somewhat answered my query. I suppose it could be argued that a different sort of mental test might exhibit an increase in rate of oxygen consumption, but I agree that the Stroop task is pretty mentally demanding.

I have done only some in-house testing, not enough for publication. I think that negative results would not be an obstacle for publication in this particular instance. I just need to do a proper study, but I have other ones planned at the moment. If you can find an even more demanding mental task to test, go ahead. It would be an interesting investigation