Source of sensory data related to cognitive effort

Question

How does the brain assess cognitive effort? Is there a chemical that is being created or consumed at a rate that is proportionate to the amount of effort felt?

I'm mostly interested in short term effort, such as in short-term-memory tasks.

Answer 1

I assume this is a question about perceived mental effort, which is closely related to (though not the same as) cognitive load, and is a specific kind of introspection, or more generally metacognition. In general, introspection tends to rely on self-perceived heuristics, rather than direct insight of mental process, and this is likely the case for mental effort. Scheiter, Ackerman, & Hoogerheide (2020) summarize as follows:

... in metacognitive research, it is well established that people do not have access to their actual knowledge. They assess their chance of success by utilizing heuristic cues which are both experience-based (gut feeling, mainly based on fluency) and theory-based (based on beliefs and lay theories). Referring to effort appraisals as metacognitive judgments brings to the fore the heuristic cues people may utilize to infer their effort assuming that they cannot sense reliably their effort.

The actual heuristic cues used in this particular case are unclear, but there are some good leads. The related metacognitive assessment of task difficulty - known as cognitive ease or "fluency" as quoted above - appears to use latency as a heuristic. That is, how long it takes to complete a mental process is indicative of its difficulty. This heuristic could also be useful for judging mental effort (eg, Moore & Picou, 2018), but not in a straightforward way: Longer tasks tend to require more effort, but also allocating more effort makes tasks take less time. Thus, how task difficulty influences effort appraisals may depend on how subjects interpret the relationship between effort and difficulty.

Accordingly, research findings suggest that while in many cases difficult (longer) tasks are associated with more effort, in others task difficulty is interpreted to indicate engagement, so difficult (longer) tasks are the result of less effort expended (Miele, Finn, & Molden, 2011; Koriat, Nussinson, & Ackerman, 2014; Fisher & Oyserman, 2017; Koriat, 2018; also see meta-analysis by Baars et al, 2020), depending on personality, instructions, and context. Other heuristic cues may also affect effort appraisals, such as positive or negative feedback (Raaijmakers et al, 2017), and performance incentives (Garrison, 2020).