Neural mechanisms underlying the effects of physical fatigue on effort-based choice (2020) Hogan et al

[Colin]

Physical fatigue crucially influences our decisions to partake in effortful action. However, there is a limited understanding of how fatigue impacts effort-based decision-making at the level of brain and behavior. We use functional magnetic resonance imaging to record markers of brain activity while human participants engage in uncertain choices for prospective physical effort, before and after bouts of exertion. Using computational modeling of choice behavior we find that fatiguing exertions cause participants to increase their subjective cost of effort, compared to a baseline/rested state. We describe a mechanism by which signals related to motor cortical state in premotor cortex influence effort value computations, instantiated by insula, thereby increasing an individual’s subjective valuation of prospective physical effort while fatigued. Our findings provide a neurobiological account of how information about bodily state modulates decisions to engage in physical activity.

Open Access: nature communications

Popular article: Got fatigue? Study further pinpoints brain regions that may control it

 

[Snowdrop]

A possibly relevant discussion on defining fatigue on this thread:

https://www.s4me.info/threads/sympt...for-us-2020-wilshire-et-al.16482/#post-283689

 

[spinoza577]

It would be more interesting, of course, to have a mechanism underlying fatique itself.

 

[Snowdrop]

Now that I've skimmed the two links it looks very much like this has nothing to do with what people with ME experience but fatigue in normal healthy individuals. Could have missed something here I suppose. But it doesn't seem relevant to the type of 'fatigue' (a not very accurate word really) that is experienced with ME.

 

[Snow Leopard]

The Science daily article is a bit confused.

The results are largely unsuprising. They simply associated lower activity of some premotor regions with less compensatory effort during fatiguing activity.

The sense of effort is upstream of the motor cortex and is effectively the level of upstream drive that goes into the motor cortex. The motor cortex itself does not sense effort. When the excitability of the motor cortex is lowered (which is noted) due to feedback from fatigue sensing afferents, this is "central fatigue" and means lower force output at the same effort level.

The authors are noting that those who 'felt the most fatigue' had less behavioural change UPSTREAM from the motor cortex (which they noted as associated with the dorsal anterior cingulate and bilateral insula - premotor regions). These participants didn't increase their effort to compensate, but why is this surprising at all?

The authors state:

In the context of our experiment, if an individual’s motor system does not appropriately adjust its resting state in response to repeated exertions, one might feel that effort is particularly costly because of the discrepancy between the motor production that one believes they can achieve and their actual motor capacity following fatigue. In contrast, it is possible that fatigue arises from an accurate representation of an individual’s bodily state and that changes in subjective preferences are simply a reflection of the altered motor cortical state that occurs following fatiguing physical exertion11,12,13,14,15.

These findings are consistent with the idea that a miscalibration of motor regions is related to fatigue-induced changes in subjective effort preferences—those participants that find effort to be particularly costly following fatigue may be those who do not modify their motor cortical activity to accommodate the reduced motor capacity that results from repeated exertion.

The authors are tying themselves in knots that don't need to exist, there doesn't have to be any miscalibration at all. The subjective increase in effort for the same motor output is due to the attenuation of motor excitibility due to afferent feedback. So there is a very real requirement for more effort and if they are feeling fatigue, some choose not to put in that extra effort hence the lower activity in the premotor areas.