Neuroscientists have successfully increased the motivation to exert mental effort by using a weak alternating electrical current sent through electrodes attached to the scalp to synchronize brain waves. The findings, published in Cognitive, Affective, & Behavioral Neuroscience, help to identify the neural mechanisms underlying the willingness to engage in mental effort, suggesting that midfrontal theta oscillations play a key role.
“For a long time research has mainly focussed on which brain mechanisms underlie mental processes, but in the recent years it has become clear that engaging in mental activities needs to be understood as an active decision process where humans are willing to perform demanding mental tasks only if they are ‘worth it.’ The goal of our research was to get a better understanding of the brain mechanisms causally determining our motivation to engage in demanding mental activities,” explained study author Alexander Soutschek, a research group leader at the psychology department of the University of Munich
For their study, the researchers utilized transcranial alternating current stimulation (tACS), a non-invasive neurostimulation technique that applies low-amplitude electrical current to the scalp through electrodes. The current modulates the neural activity in the brain regions under the electrodes, potentially enhancing or suppressing specific cognitive processes.
The researchers used tACS to induce a type of rhythmic neural activity known as midfrontal theta oscillations. Midfrontal theta activity has been associated with top-down regulation of attention, task preparation and switching, and the maintenance of goal-directed behavior. But it was unclear whether midfrontal theta oscillations were causally involved in cost-benefit calculations.
In the study, 35 healthy young volunteers (age range 19-33) received tACS or a sham stimulation over the dorsomedial prefrontal cortex while deciding whether to perform harder or easier versions of the N-back task for differing levels of monetary rewards.
In the task, participants were presented with a sequence of items (in this case, letters) and are asked to indicate if the current item matches the item presented “n” steps back in the sequence. The “n” value specifies the difficulty of the task, with larger “n” values requiring greater working memory capacity.
The researchers found that participants exhibited a greater willingness to exert mental effort for rewards under the tACS condition compared to the sham condition, providing evidence that heightened midfrontal theta oscillations causally increase goal-directed mental effort.
“Humans often have to engage in demanding mental activities in order to reach their goals,” Soutschek told PsyPost. “In the current study, we identified the neural mechanism enabling humans to decide whether a goal is worth the required mental labor.”
The brain stimulation increased the general propensity to choose high reward-high effort options. But it did not change the sensitivity to reward values or effort costs.
“Our results suggest that increasing theta oscillations in the dorsomedial prefrontal cortex enhances the motivation to perform a demanding mental task for a reward,” Soutschek explained. “We found this surprising, given that theoretical accounts assume that this region should be involved in trading-off effort costs against the rewards at stake rather than generally increasing the motivation for rewarded mental effort.”
But the study, like all research, includes some caveats.
“While the simulations of the electrical field induced by the brain stimulation intervention suggests that the effect of the stimulation are strongest in the brain region we targeted (dorsomedial prefrontal cortex), the current data do not allow deciding whether the influence of the stimulation on behavior is indeed caused by stimulation effects on the anterior cingulate cortex or any other brain region,” Soutschek said.
“This question can only be answered by combining brain stimulation with functional neuroimaging, which we plan to do in the future to get a better understanding of how our neural intervention changed brain functioning.”
The study, “Brain stimulation over dorsomedial prefrontal cortex modulates effort‑based decision making“, was authored by Alexander Soutschek, Lidiia Nadporozhskaia, and Patricia Christian.