Reward sensitivity following boredom and cognitive effort: A high-powered neurophysiological investigation

Highlights

• A boredom induction led to more subjective fatigue than a cognitive effort condition.

• Reward sensitivity larger following boredom condition than control condition.

• Reward sensitivity following cognitive effort did not differ from boredom nor control.

• No relation between self-report fatigue and reward sensitivity.

Abstract

What do people feel like doing after they have exerted cognitive effort or are bored? Here, we empirically test whether people are drawn to rewards (at the neural level) following cognitive effort and boredom. This elucidates the experiences and consequences of engaging in cognitive effort, and compares it to the consequences of experiencing boredom, an affective state with predicted similar motivational consequences. Event-related potentials were recorded after participants (N = 243) were randomized into one of three conditions – boredom (passively observing strings of numbers), cognitive effort (adding 3 to each digit of a four-digit number), or control. In the subsequent task, we focused on the feedback negativity (FN) to assess the brain's immediate response to the presence or absence of reward. Phenomenologically, participants in the boredom condition reported more fatigue than those in the cognitive effort condition, despite reporting exerting less effort. Results suggest participants in the boredom condition exhibited larger FN amplitude than participants in the control condition, while the cognitive effort condition was neither different from boredom nor control. The neural and methodological implications for ego depletion research, including issues of replicability, are discussed.

Introduction

Imagine you just spent the morning grading exams for a large course. It was boring work, and you feel drained. What do you feel like doing? Relaxation might immediately come to mind, but what if you have to get back to work? You may find other ways of rewarding yourself – perhaps eating a chocolate bar, allowing yourself a few minutes to peruse Facebook, or generally engaging in some other activity that you find pleasurable or rewarding. Although people generally seek out rewarding experiences, here we wonder if this is especially the case after being bored or engaging in cognitive effort. In the present study, we empirically test whether people are more drawn to rewards after boredom or after engaging in cognitive effort compared to when they are neither, by examining the sensitivity to rewards on a neural level.

Neurocognitive accounts of cognitive effort view it as the mobilization of resources necessary to attain a desired level of performance (Shenhav et al., 2017). The term cognitive effort is frequently used interchangeably with cognitive control, with some suggesting that cognitive effort drives the decision to engage control (Westbrook and Braver, 2015). Here, we define cognitive effort as the intensification of control, usually brought about by highly demanding or difficult tasks. Cognitive effort, and in particular decisions about engaging effort, is thought to be neurally mediated by the dorsal anterior cingulate cortex (dACC) and the lateral prefrontal cortex (lPFC; see Shenhav et al., 2017, for review). Importantly, cognitive effort is generally considered inherently aversive or costly (Kool et al., 2010, Westbrook et al., 2013; but, see Inzlicht et al., 2018). According to some recent models, this cost is then weighed against the benefits of exerting the effort. Although there are some differences in the accounts of why mental effort is costly (see Shenhav et al., 2017, for a review), one proposed suggestion relates to the notion of opportunity costs (Kurzban et al., 2013). When engaging in cognitive effort on any given task, a person foregoes opportunities to engage in other (potentially valuable) tasks – these lost opportunities are termed the opportunity cost of persisting at an effortful task. According to this model, the costs of engaging in further effort are expected to rise as more effort is exerted, with the value of effort exertion (i.e., the cost-benefit ratio) diminishing proportionally (Kurzban et al., 2013). After incurring large effort costs, people may want a (proportionally large) reward.

The consequences of exerting cognitive effort have also been the focus of a large body of literature on ego depletion. Ego depletion refers to a psychological state whereby people feel unable or unwilling to exert effort following an effortful task. It is akin to a state of mental fatigue (Inzlicht et al., 2014), whereby after engaging in an activity that requires effortful control, people perform more poorly on a second task, also requiring effortful control (Baumeister et al., 1998). This sequential ego depletion paradigm (consisting of two sequential tasks requiring cognitive effort) has been used in hundreds of studies (see Hagger et al., 2010), although a recent pre-registered replication report did not find an effect for one specific operationalization of depletion (Hagger et al., 2016).

While the very existence and magnitude of the ego depletion effect are currently being examined (Carter and Mccullough, 2014, Hagger et al., 2016, Friese et al., 2018), some have wondered how replication difficulties are affected by the possible mechanisms underlying ego depletion (Inzlicht et al., 2014, Kurzban et al., 2013). One explanation for the “depletion” period frequently observed after a demanding self-control task centers on motivation – after exerting effort on one task, people are no longer motivated to exert further effort on a subsequent task, and instead prefer to ‘indulge’ in an immediate temptation (e.g., of slacking off, venting one's anger, eating the delicious food, etc.). According to this shifting priorities model of self-control (Inzlicht et al., 2014, Milyavskaya and Inzlicht, 2017), this shift in motivation would be accompanied by shifts in attention, perception, emotion, and memory. Thus after an effortful task, people may be more drawn to rewards, and may more readily notice opportunities to indulge in rewarding behaviours. Fatigue is thought to have similar motivational consequences (Hockey, 2013; but, see Gergelyfi et al., 2015). Indeed, some have argued that fatigue and depletion refer to the same phenomenon (Inzlicht and Berkman, 2015), and in the present manuscript we use fatigue and depletion interchangeably.

For example, participants are more likely to gamble (Bruyneel et al., 2009), shop (Vohs and Faber, 2007), eat (Vohs and Heatherton, 2000), and smoke (Shmueli and Prochaska, 2009) after engaging in cognitive effort. In another study, exerting cognitive control for an extended period led participants to prefer smaller yet immediate monetary rewards instead of larger delayed rewards; this preference in immediate rewards was also linked to a reduced activation of the lateral prefrontal cortex (LPFC; Blain et al., 2016). Similarly, an experience-sampling study found that people were more likely to succumb to tempting desires when fatigued from repeated use of self-control throughout the day (Hofmann et al., 2012). However, since enactment of a temptation is jointly determined both by the strength of the desire and the amount of effort exerted (Hofmann et al., 2012), such effects could occur either because participants are more sensitive/tempted by the rewards (i.e., the desire is greater; Schmeichel et al., 2010), or because they exert less self-control when faced with the desire.

To our knowledge, only a few studies have examined the effects of ego depletion or the exertion of cognitive effort on the perception of rewards. In one study, participants who were depleted were more accurate in detecting a reward-related symbol ($) than non-reward symbol (&) in rapidly presented images (Schmeichel et al., 2010). Using a much longer induction of cognitive fatigue (over 6 h of cognitive control tasks) and a delay discounting task, Blain and colleagues (2016) found an increase in preference towards more immediate monetary rewards (instead of delayed but larger rewards). In another particularly relevant study, dieters who were depleted exhibited greater food-cue-related activity in areas of the brain associated with coding reward values, and other areas associated with self-control (Wagner et al., 2013). In that study, chronic dieters either completed a depletion task or a control task and then viewed desirable foods while in an fMRI scanner. Participants in the depletion condition had greater activation in the orbitofrontal cortex, as well as lesser functional connectivity between the orbitofrontal cortex and the inferior frontal gyrus, than those in the control condition (Wagner et al., 2013). Together, these studies support the possibility that after people exert cognitive effort, they exhibit increased sensitivity towards rewards.

If the consequences of effort expenditure affect states of motivation, then depletion might have similar motivational properties as other states that influence motivation towards rewards. One such state is boredom. Boredom is typically described as an affective state that results from the inability to “successfully engage attention with internal or external information” (Eastwood et al., 2012, pg. 484); it is characterized by “core motivational deficits accompanied by a phenomenological experience of a lack of interest or affective engagement.” (Goldberg et al., 2011, pg. 649). Here, we similarly define boredom as a state produced by under-stimulation, where desires for stimulation and engagement are not being met.

Although it may at first seem paradoxical that engaging in cognitive effort would lead to the same consequences as boredom, there are several reasons to theorize that boredom might have similar motivational consequences to depletion. First, research on vigilance tasks that require participants to monitor displays for infrequent stimuli for a prolonged period of time (e.g., Mackworth, 1948) have been alternatively interpreted as inducing fatigue (i.e., depletion) or boredom (Pattyn et al., 2008). Indeed, research has shown that both the “depletion of information-processing resources” (i.e., an overload of the attentional system; Pattyn et al., 2008, pg. 377) and under-arousal both lead to a decrease in vigilance (see Pattyn et al., 2008). In other words, vigilance tasks might induce both (1) fatigue and (2) boredom, both of which might have similar downstream consequences on subsequent behaviour.

Second, animal models of boredom find that animals housed in cages with no opportunities for enrichment display more interest in novel stimuli and consume more food rewards (Meagher and Mason, 2012). That is, these ‘bored’ animals are more attuned to rewards. Similarly, in humans greater and more frequent experiences of boredom have been linked to engaging in more impulsive, reward-seeking behaviour including gambling (Blaszczynski et al., 1990), overeating and binge eating (Abramson and Stinson, 1977, Myhre et al., 2015), and alcohol and drug abuse (Iso-Ahola and Crowley, 1991); this is similar to the impulsive tendencies of depleted participants (e.g., Vohs and Heatherton, 2000; Vohs and Faber, 2007). While research finds that people who are generally prone to boredom engage more frequently in such impulsive behaviours, to our knowledge there has not been any research examining whether state boredom would directly affect a person's orientation towards rewards.

One proposed function of boredom is that boredom serves as an indicator to pursue an alternate goal (Bench and Lench, 2013). A similar motivational function of depletion has also been proposed, with depletion or fatigue seen as a stop-signal, a signal to end cognitive effort and engage in other pursuits (Hockey, 2013, Inzlicht et al., 2014, Kurzban et al., 2013). This suggests that boredom and depletion might have similar functions in orienting humans to disengage from current behaviour and seek other (more rewarding) alternatives. A purpose of the current study, then, is to test whether boredom also elicits a stronger orientation towards rewards. Specifically, we predicted that participants who were either bored or depleted would show an increased sensitivity to rewards compared to participants who were neither depleted nor bored.

In the present study, we looked at reward sensitivity as the brain's immediate responses to reward using electroencephalographic (EEG) recordings, focusing on the feedback negativity (FN) component of the scalp-recorded event-related potential (ERP). The FN is a negative deflection in the ERP at frontocentral electrode recording sites that occurs around 250 ms after feedback presentation and is larger (i.e., more negative) to unfavorable than favorable outcomes. Recent research (see Proudfit, 2015 for review) suggests that the FN may represent a positive response to positive feedback (reflecting gains or rewards) that is reduced following negative feedback, with the difference between negative and positive feedback (negative minus positive) representing the negative deflection seen in the FN. In this view, the FN is actually a decrease in the positivity associated with reward/favorable feedback (Proudfit, 2015), rather than a negative deflection per se. Nonetheless, The FN has good psychometric properties (Levinson et al., 2017) and has been correlated with subjective interest in rewards (Bress and Hajcak, 2013), and with approach motivation more generally (Threadgill and Gable, 2016). In the present study, we examine the FN to evaluate the extent to which depleted and bored participants are drawn towards rewards.

We conducted a high-powered study to examine the effects of effort expenditure and boredom on reward sensitivity. We also included measures of phenomenology as a manipulation check, expecting that the effort condition would result in high self-reported effort and fatigue (compared to the boredom condition), and that the boredom condition would lead to greater boredom (compared to the effort condition). For our main research questions, we first hypothesized that participants who exert cognitive effort (i.e., the effort condition) would have a greater sensitivity to rewards (as indexed by the FN) than participants in the control condition. Importantly, we expected the boredom condition to have an effect similar to the effort condition, with both resulting in higher sensitivity to rewards than the control condition. We also conducted exploratory analyses to examine whether the type of reward mattered. Specifically, previous research has found that monetary rewards elicited greater FN than no-value rewards; while we expected to replicate this main effect, we did not have specific predictions as to whether there would be an interaction with condition.