Isolating Neural Reward-Related Responses via Pupillometry

Recent research has shown that reward-related stimuli capture attention in an automatic and involuntary manner, or reward-salience (Le Pelley, Pearson, Griffiths, & Beesley, 2015). Although patterns of oculomotor behavior have been previously examined in recent experiments, questions surrounding a potential neural signal of reward remain. Consequently, this study used pupillometry to investigate how reward-related stimuli affect pupil size and attention. Across three experiments, response time, accuracy, and pupil were measured as participants searched for targets among distractors. Participants were informed that singleton distractors indicated the magnitude of a potential gain/loss available in a trial. Two visual search conditions were included to manipulate ongoing cognitive demands and isolate reward-related pupillary responses. Although the optimal strategy was to perform quickly and accurately, participants were slower and less accurate in high magnitude trials. The data suggest that attention is automatically captured by potential loss, even when it is counterintuitive to current task goals. Regarding a pupillary response, patterns of pupil size were inconsistent with our predictions across the visual search conditions. We hypothesized that if pupil dilation reflected a reward-related reaction, pupil size would vary as a function of both the presence of a reward and its magnitude. More so, we predicted that this pattern would be more apparent in the easier search condition (i.e., cooperation visual search), because the signal of available reward was still present, but the ongoing attentional demands were significantly reduced in comparison to the more difficult search condition (i.e., conflict visual search). In contrast to our predictions, pupil size was more closely related to ongoing cognitive demands, as opposed to affective factors, in cooperation visual search. Surprisingly, pupil size in response to signals of available reward was better explained by affective, motivational and emotional influences than ongoing cognitive demands in conflict visual search. The current research suggests that similar to recent findings involving LC-NE activity (Aston-Jones & Cohen, 2005; Bouret & Richmond, 2009), the measure of pupillometry may be used to assess more specific areas of cognition, such as motivation and perception of reward. However, additional research is needed to better understand this unexpected pattern of pupil size.