Research ReportInhibition of the supplementary motor area affects distribution of effort over time
Introduction
People often face a need to exert effort through an extended period of time, until they achieve a goal or reach a deadline. For example, employees work until the end of their shift, and video-game players exert effort until they complete a quest. In many such situations, people tend to invest high levels of effort in the beginning and towards the end of the task, and lower levels of effort in its middle. This U-shape pattern of effort allocation has been termed “the stuck-in-the-middle effect” (STIM) and was demonstrated in both laboratory and field behavioral studies (e.g., Bonezzi et al., 2011; Touré-Tillery & Fishbach, 2012, 2015). For example, Bonezzi et al. (2011) had participants correct typos in a series of nine essays. They found that participants corrected typos faster in the 2nd and 8th essays compared to the 5th. As another example, athletes in track-races, swimming, rowing and cycling were found to demonstrate a STIM pattern through their bouts, performing the first and last intervals faster than the middle ones (Foster et al., 2004; McGibbon et al., 2018; Muehlbauer et al., 2010; Tucker et al., 2006).
In general, level of effort is believed to reflect a balance between effort cost and its reward (e.g., Kool & Botvinick, 2014; Morel et al., 2017; Shenhav et al., 2017; Westbrook & Braver, 2015), and by that account, the STIM pattern would reflect higher reward and/or lower cost for effort in the beginning and the end of a task. Consistent with this view, it has been suggested that effort cost increases over time (and correspondingly effort decreases in the course of a task; Kurzban et al., 2013), and that actions are experienced as more rewarding in the beginning and the end of a task, giving rise to the STIM pattern (Bonezzi et al., 2011; Heath et al., 1999; Katzir et al., 2020). Although much is known about the psychological causes of the STIM effect, its neuronal underpinnings remain unknown. The present study aimed to close this knowledge gap, and thereby enrich our understanding of effort allocation processes in general and the STIM pattern in particular.
Recently, non-invasive supplementary motor area (SMA) inhibition has been implicated in perceiving effort as lower. Specifically, Zénon, et al. (2015) found that participants reported lower effort and showed less pupil-dilation (a physiological index of effort) while squeezing a handgrip, following inhibitory continuous theta burst stimulation (cTBS) over the SMA compared to a control precuneus stimulation. In addition, participants accepted offers to reproduce an effortful squeeze in return for a given payment more after cTBS to the SMA than to a control location. This latter finding is also consistent with the possibility that inhibition of the SMA caused participants to perceive the level of effort that they exerted as lower. Importantly, the SMA was not implicated in reward-processing per se, which has been found to be associated with activity in other regions such as the orbitofrontal cortex (OFC), dorsolateral prefrontal cortex (dlPFC), ventromedial prefrontal cortex (vmPFC), dorsal anterior cingulate cortex (dACC), and the striatum (e.g., Bonnelle et al., 2015; Burke et al., 2013; Croxson et al., 2009; Lee et al., 2007).
In light of these findings, we hypothesized that inhibiting SMA activity would modulate the STIM pattern of effort allocation. Specifically, in light of the results of (Zènon et al., 2015), we hypothesized that stimulation to the SMA would make participants perceive effort as less costly. We thought that this would make levels of effort less evenly distributed in the course of the task, and more responsive to proximity to the beginning and the end of the task, and thus predicted a steeper, more pronounced STIM pattern of effort allocation after stimulation to the SMA compared to a control location.
We employed a simple computer game, which we validated in three control experiments and a pretest as a procedure that enables measurement of effort and gives rise to the STIM pattern of effort allocation. In this game, participants control a spaceship and shoot asteroids using the spacebar, with the number of spacebar presses serving as the measure of effort (see Ames & Fiske, 2015 and Control Experiment 1 for a validation of this measure). Participants completed the game after undergoing inhibitory 1 Hz repetitive transcranial magnetic stimulation (rTMS) over the SMA or a control precuneus location (see Materials and Methods).
Section snippets
Participants
Twenty-one healthy, right-handed volunteers with no neurological or psychiatric history participated in the study (10 female; Mage = 25, SDage = 3.2). Sample size was determined based on Zénon et al.’s (2015) findings, which indicated over 80% statistical power for the main effect of TMS stimulation site on effort-perception, both in self-report ratings and pupil dilation in a 3 (stimulation condition: M1/SMA/precuneus) X 4 (effort intensity: 10/23/37/50% of maximal voluntary contraction)
Confirming the vertex of the effort distribution pattern
We expected both conditions to exhibit a STIM pattern of effort distribution in the course of the task, which is essentially a U-shape dependency of level of effort on time. After an initial inspection of this dependency, which confirmed a U shape (Fig. 2, see also formal analyses below) we sought to determine the vertex of the STIM pattern. Specifically, as the pretest identified the vertex at Time-Segment 3, we sought to further validate this segment as the vertex also in the present
Discussion
We investigated the neural mechanisms underlying patterns of effort allocation, and the “stuck-in-the-middle” (STIM) effect specifically, whereby humans invest effort at the beginning and the end of prolonged tasks, yet reduce effort in the middle. Effort distribution was measured following inhibitory rTMS over the SMA or a control site. The results showed that inhibitory SMA stimulation resulted in a more pronounced STIM effect compared to control stimulation. In addition, a control analysis
Credit author statement
A.E., J.H., H.S., N.L. and N.C. designed research and experimental protocol; A.E. and J.H. performed research; A.E. analyzed data; A.E., N.L., J.H. and N.C. wrote and edited the manuscript.
Funding
This work was supported by the I-CORE Program of the Planning and Budgeting Committee and the ISF (grants 51/11 and 526/17), Israel.
Open practices
The study in this article earned Open Materials, Open Data and Preregistered badges for transparent practices. Materials and data for the study are available at https://bit.ly/2KtbSbn.
Declaration of competing interest
The authors declare no competing financial interests.
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