Marshmallows and Parolees

Marshmallows and Parolees

Perhaps you remember the marshmallow experiment – placing an elementary school child in a room with a delicious marshmallow as a test of their willpower. If they leave the marshmallow to sit for just five minutes, they are given two marshmallows. I suspect that fewer of you remember a study on how our cognitive strength falters during the day. The study considered the decisions made by a parole board on prisoner requests for early release. Here is what they found:

Ordinal position refers to the time of day, early to the left, late to the right. Those circles reflect breaks taken by the parole judge for snacks and lunch, lasting about a half-hour. As you can see, your best chance of being paroled was early in the day or just after a meal and break. The researchers mused that:

“Executive function can be restored and mental fatigue overcome, in part, by interventions such as viewing scenes of nature, short rest, experiencing positive mood, and increasing glucose levels in the body.”

A new study suggests cognitive load is not like a “willpower muscle,” where exercise does not make it stronger, nor is it due specifically to a short rest or moment in nature. The current study suggests that it is all about glutamate.

Why is focused thinking hard?

We have all experienced fatigue after we spend significant time focused on a project that requires “a great deal of thought.” In the marshmallow experiment, researchers thought it might be due in part to our willpower to focus and that, like a muscle, that repetitive focus would strengthen that ability to sustain critical thinking. The experience of the parole board suggested that a break was needed. In this current study, the researchers hypothesize that sustained critical thinking is all about energetics, specifically the energetics of areas of the brain associated with deep thinking.

The Study

Cognitive fatigue was created by presenting a variety of stimuli to the participants. In one case, asking them to identify the letter as either a vowel or consonant – increasing the cognitive load created by shortening the period between letters. In the second, asking them to determine whether a letter was seen in “n” presentations previously – where the increased cognitive load was based on a larger "n."

The researchers used two biological measures. They considered the energetics of the inferior pre-frontal cortex, IPFC, an area in the front of our brains associated with executive functions like planning, short-term memory, and decision-making. They used functional MRI to look at levels of the metabolic product of IPFC energy flows. While they looked at several metabolites, glutamate was altered the most by cognitive work. Parenthetically, glutamate is a more specific source of energy than the glucose levels alluded to by the parole researchers. It was unclear whether glutamate might be depleted through “concentration” or build up in excess and “turn off” focus.

The second biological measure was the pupillary size which evidently dilates as with put effort into thinking. The researchers measured the time from the initial presentation of the task, when the pupil is constricted, to the time when the pupil dilates, signifying deep thought.

Greater cognitive effort as measured by those pupillary responses correlated with more glutamate in the IPFC area of the brain, consistent with an accumulation rather than depletion mechanism controlling mental effort.

“…higher-demand cognitive control tasks [are] associated with greater glutamate release, which would result in steeper glutamate accumulation with time on task across a workday.”

Glutamate is an excitatory neurotransmitter and is tightly regulated. Glutamate usually is more concentrated intracellularly, where it serves as both a precursor for proteins and as a means to “detoxify” ammonia. These studies revealed high extracellular levels, which simultaneously diminishes its intracellular role, while extracellular accumulation alters the balance of excitation and inhibition towards more excitability – less control.

Sleep has been associated with a reduction in extracellular glutamate, so it is not a great leap to conclude that short rest periods would diminish these accumulations. That would certainly fit the picture of the parolees and the parole judges. Glutamate may not be the signal; it might be anywhere along its metabolic path. But what seems clear is that the flow of energy influences our perception of fatigue, even when the perception is related to our thoughts rather than deeds.

Two take-home thoughts.

• That tiny voice in our head, our self, is in part biochemical kinetics but is so much more than the sum of the parts.
• Being the first case is best if you are up for parole or sentencing.

Source: A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions Current Biology DOI: 10.1016/j.cub.2022.07.010