Investigating the Interaction of Reward and Emotion Using Emotional Faces as Cues in the Monetary Incentive Delay Task.

In our daily life, numerous behaviors are driven and influenced by motivation. Reward, a motivational factor defined as a stimulus that induces approach behavior and increases its future occurrence (Lutz & Widmer, 2014), plays a crucial role in learning, working, and social interactions. Previous researches frequently employ the monetary incentive delay (MID) paradigm (Knutson et al., 2000) to investigate reward processing and its impact on cognitive processing. In this paradigm, a cue signals potential rewards contingent on task performance, which enhances the participant’s motivation to perform the task. Typically, these cues are emotionally neutral. However, real-life scenarios often involve emotionally charged contexts, such as a mother smilingly promising a trip for good grades or a nurse reassuringly smiling before an injection. Moreover, reward and emotion are tightly intertwined theoretically. Reward typically elicits positive emotion and emotional states influence reward perception and response (Berridge & Robinson, 2003). Neuroanatomically, there is overlap in brain regions involved in reward and emotional processing, including the amygdala, hippocampus, ventromedial prefrontal cortex, and midbrain dopamine system (Murray, 2007; Singer & Frank, 2009; Wise, 2004; Burgdorf & Panksepp, 2006). Conceptually, the core dimensions of reward include value and salience, while the core dimensions of emotion encompass valence and arousal. Thus, based on previous research, two hypotheses can be proposed regarding the interaction between reward and emotion: the Valence Congruency Hypothesis and the Salience Hypothesis. The Valence Congruency Hypothesis suggests that positive and negative systems in certain brain regions are organized as opponent systems in a push-pull manner (Konorski, 1967; Solomon & Corbit, 1974), potentially leading to the mutual suppression and competition of their responses. According to this hypothesis, the congruency between reward and emotional valence would result in superior performance compared to incongruent conditions. This aligns with previous findings from emotion categorization tasks, where participants categorized positive words faster than negative words under gain-prospect reward cues and vice-versa under loss cues (Derryberry, 1988). Another recent study by Park et al. (2019), which explicitly signaled the reward prospect using one of three emotional facial expression categories (angry, happy, or neutral), observed that response times were facilitated when happy faces signaled the reward prospect and slowed down when angry faces signaled the reward prospect. These findings also support the idea of the Valence Congruency Hypothesis. Moreover, studies have found that reward cues can counteract the influence of task-irrelevant negative stimuli, such as distracting negative images (Padmala & Pessoa, 2014) or threat-related stimuli (Choi et al., 2014, 2015; Wei et al., 2016), further illustrating the competitive relationship between different valences. However, the Salience Hypothesis posits that reward and emotion are represented by their salience (Chakravarthula & Padmala, 2022). That is to say, when reward and emotional information are presented simultaneously, regardless of their valence, the increased salience of the stimuli enhances task performance. For instance, Walsh and colleagues employed a letter-search task in an array displayed above and below a central distractor stimulus that was either negative, positive, or neutral. They reported that reward prospects mitigated the interference effect from both positive and negative emotional distractors to a similar extent (Walsh et al., 2018). Moreover, this enhancement is also reflected at the neuroanatomical level: a separate study showed an additive pattern of activation in the caudate and ACC for the reward-plus-threat condition (Hu et al., 2013). The mixed findings regarding the interaction of reward and emotion so far might be attributable to their differences in the experimental task designs, emotional materials (such as images or faces), and variations in the type of negative emotions (e.g., anger vs. fear). More importantly, in most of these studies, reward information takes precedence over emotional content, rendering the emotional information secondary and needing to be suppressed to complete the task and obtain the reward (but see Park et al., 2019). This discrepancy in task relevance may lead to differential processing of reward and emotional information, thereby introducing bias into research on the interaction between reward and emotion. Consequently, our understanding of how reward and emotion interact when both types of information are equally relevant to the task remains limited. What’s more, since previous studies using the Monetary Incentive Delay (MID) task to investigate reward processing did not include emotionally salient reward cues, very little is known regarding the mechanisms underlying the interaction between reward anticipation and emotional information, which is of both basic and clinical relevance (Rutledge et al., 2017). Therefore, this study adopts the traditional MID paradigm by using emotional faces (positive, negative, and neutral) as incentive cues to indicate potential monetary gains or losses. We set up two conditions: valence-congruent and valence-incongruent. The valence-congruent condition refers to using positive emotional faces to indicate gains and negative emotional faces to indicate losses. In contrast, the valence-incongruent condition refers to using positive emotional faces to indicate losses and negative emotional faces to indicate gains. By setting up these two conditions, we aim to investigate how reward and emotion interact during information integration when both are equally relevant to the current task and test the validity of the Valence Congruency Hypothesis and the Salience Hypothesis.