Successful goal-directed behavior often requires the exertion of effortful control processes. The striatum is thought to play a key role in determining whether a goal is worth the required cognitive effort, but whether and how the striatum causally influences effort-based decisions in humans remains unclear. Here, we address this gap by employing transcranial temporal interference stimulation (tTIS), a novel non-invasive brain stimulation technique capable of targeting deeper brain regions.
In particular, we applied striatum-targeted tTIS to healthy participants performing an effort-based decision task during functional MRI scanning. Supporting the idea that the striatum encodes both the benefits and costs of actions, we found that, on the behavioral level, striatum-targeted tTIS increased the sensitivity to both reward magnitudes and effort costs. At the neural level, this was mirrored by stronger representations of effort demands in the striatum under stimulation as well as by enhanced functional coupling between the striatum and the anterior cingulate cortex, a region at the intersection of motivation and cognition.
Together, our findings provide insights into how striatum-targeted stimulation influences trade-offs between rewards and effort costs, informing neural accounts of motivated cognition and suggesting novel neural interventions for the treatment of amotivation. From learning for an exam to writing scientific articles, successful goal-directed behavior often requires cognitive control processes. However, people often avoid mentally demanding activities despite their beneficial consequences [1, 2], suggesting that they trade off the benefits against the costs of effortful cognition.
Moreover, pathologically reduced motivation for mental effort is a core symptom of several clinical conditions [3,4,5] and may contribute to the cognitive impairments in these disorders [6, 7]. For both basic and clinical research, it is therefore important to obtain a better understanding of the neural mechanisms underlying the motivation for mental effort. Deciding whether a goal is worth the required effort costs is thought to be implemented by a distributed neural network in which the striatum plays a key role [8,9,10,11,12].
The striatum encodes both reward magnitudes and the costs of actions, including cognitive effort [11, 13], in the direct and indirect dopaminergic pathways, respectively. Recent models propose that the relative balance of the direct and indirect striatal pathway implements a cost-control signal that regulates the perceived affordability of mental effort relative to the value of the reward at stake [14,15,16]. Activation in the direct and indirect pathway in turn influences neural processing of cost-benefit trade-offs in prefrontal regions such as the dorsal anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (dlPFC), which evaluate control demands and integrate cost-benefit information [17, 18].
The ACC in particular was ascribed a central role in deciding whether to engage in effortful control (implemented by the dlPFC) for goal achievement [19, 20]. Via its structural connections with the striatum, the ACC may trade off reward against cost signals provided from the striatum to decide about control allocation [21,22,23,24]. Despite the hypothesized role of the striatum for motivating mental effort, however, we have only a limited understanding of how activation in the striatum causally influences decisions to engage in goal-directed mental effort.
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