Preprint Neural mechanisms underlying the effects of cognitive fatigue on physical effort-based choice 2024 Dryzer and Chib

Abstract

Fatigue is a state of exhaustion that influences our willingness to engage in effortful tasks. While both physical and cognitive exertion can cause fatigue, there is a limited understanding of how fatigue in one exertion domain (e.g., cognitive) affects decisions to exert in another (e.g., physical).

We use functional magnetic resonance imaging (fMRI) to measure brain activity while human participants make decisions to exert prospective physical effort before and after engaging in a cognitively fatiguing working memory task. Using computational modeling of choice behavior, we show that fatiguing cognitive exertion increases participants’ subjective costs of physical effort compared to a baseline rested state.

We describe how signals related to fatiguing cognitive exertion in the dorsolateral prefrontal cortex influence physical effort value computations instantiated by the insula, thereby increasing an individual’s subjective valuation of prospective physical effort while cognitively fatigued. Our results support the idea of a general fatigue signal that integrates exertion-specific information to guide effort-based choice.

https://www.biorxiv.org/content/10.1101/2024.12.06.627274v1

 

I hope their work is valid. It would be very useful to be able to measure fatigue quantitatively. Even if it's costly, the quantitative results can be used to make questionnaires about fatigue more reliable.

 

Then we are talking about a completely different thing. As is tradition. My willingness to engage in effortful tasks has never changed. In fact it's been a huge problem since my capacity to do so is far below my willingness, and this is the most common problem with ME/CFS, which is about as textbook you can get in terms of fatigue and effort.

I don't think there are many definitions of fatigue that get it this wrong. Fatigue is not a signal, it's a state. It can be that there is a signal activated in that state, but it's not going to make any difference if the exertion continues into increasingly diminishing performance. Not anymore then unplugging the fire alarm will put out the fire.

What's especially ridiculous is that this has been massively attempted for decades. Every military in the world has tried all sorts of experiments to get soldiers to just push through fatigue. Never works. Everyone needs rest. The very best they managed is prolonging a lower state of performance that also sees a diminishing cognitive performance, through the use of stimulants. They sure can continue to perform at a lower level of overall performance, but even that eventually drops down into negative performance levels.

It's quite possible that the horrible training physicians go through is part of this. They learn to operate at a lower performance level, through increasing impairment, and get the entire wrong lesson about it. Even though by definition this process serves to weed out those who don't have above average stamina, especially those with health problems, creating a very strong selection effect.

 

I think the quantitation her is about as valid as a Chalder fatigue score. We have absolutely no understanding of quantitation of neural interaction in this sort of situation. Signals from MRI tell us nothing.

It is worth remembering that a lot of brain activity is inhibitory. The classic pyramidal cells that send messages out down the spinal cord to move your arms and legs send negative signals (which is why when they are damaged you have spasticity). How on earth can you quantitate relationships from MRI when positive and negative signals will both involve oxygen uptake?

 

38 participants from the Johns Hopkins community were recruited and 13 were excluded from behavioural analysis and 14 excluded from fMRI ‘for various reasons’

the task was RISKY decisions about prospective physical activity (looking up what was risky) - so sounds less like the weight was on doing the physical activity but the pressure was on calculating the risk of those decisions ie THE choice tasks were COGNITIVE in themselves and not physical or emotional?

I’m looking up how long the whole thing took but they did rounds of this alternating an nblock working memory task with the choices all displayed by them looking at a projector that was then using an angle mirror to get it to be seen whilst they were inside (?) an fMRI machine? Which are well known for being noisy and not a comfortable set up

why did these people think it was just the cognitive task that was fatiguing and not the rest of the set up being something that might make most people feel increasingly ‘unwell’?

one participant was excluded from the neuroimaging because of excessive head movement

have they not thought of the physical exertion involved with keeping still including one’s head very still for long lengths of time whilst holding a hand grip and having to stare at and answer questions and do tasks?

 

I don’t know what they think they are measuring to be honest

they start off with a training session on hand grip - where people have to do max grip and then grip and then guess what % of exertion it was and vice versa or something as if ‘to get their eye in’ on what eg 30% exertion ‘feels like’

and then the ‘choice of prospective risky physical exertion’ is that at the end 10 of their choices will be taken at random and they have 100 times to choose between:
- doing a small exertion handgrip which is certain
OR
- a 50:50 toss of the coin chance of either no exertion or max exertion

so the worst they could end up with is having to do 10max exertion handgrips that they have to get right to get their £50, but even if all ten were option 2 probability says it would be five of those.

they excluded four people for being ‘outside the parameters’ in their choices

and 4 for what sounds like ‘always picking the same choice’

which is starting to seem like you are just excluding normal choice behaviour that’s clearly quite common that didn’t fit the preferred hypothesis

I still don’t get how they can suggest this is a measure of physical fatigue?

 

Did they WATCH the dorsolateral prefrontal cortex cognitive fatigue influence the physical effort decisions made in the insula ?

and SHOW that those less cognitively fatigued (and by the sounds ‘within person’ by results of those working memory tests) displayed something special brain wise compared to those more cognitively fatigued in a temporal way?

or have the extrapolated presumptions and then theorised in their description how they think this but haven’t evidenced it?