Reward Driven Prioritization Modulates Object-based Attention in Human Visual Cortex

Most of the recent evidence suggests that stimuli that are rewarded strongly attract visual attention and consequently modulate neural activity in the visual cortex. This raises a possibility that reward and attentional systems in the brain are greatly interconnected. However, to date, control mechanisms of attentional and reward systems have been investigated independently, and the nature of this relationship remains poorly understood. To investigate the neural mechanisms of reward and attention, using event-related fMRI, we employed a variant of the Egly, Driver and Rafal paradigm complemented with three different monetary reward schedules: (i) reward delivered randomly to either the same- or different-object target; (ii) higher reward delivered to the same-object target; and (iii) higher reward delivered to the different-object target. Since the exact same visual stimuli are presented in all three experiments, any differences in neural activity can only be attributed to the reward manipulation. It was observed that reward schedule exclusively modulated activation in the early visual areas. BOLD response was enhanced for the same-object location as compared to the different-object location when reward schedule was biased toward the same-object location (same as the traditional object-based effect). On the contrary, reward schedule biased toward the different-object location reversed the traditional object-based effect, exhibiting enhanced BOLD activation for the different-object location as compared to the same-object location. Behavioral results also supported the reward-based modulation effect, as evidenced by faster RTs for object locations with higher reward, independently of whether such location was in the same- or different-object. Importantly, the magnitude of the object-based effect was not modulated by reward schedule differentially (neither behaviorally nor in BOLD response). These results indicate that reward priority exclusively guides attention, and suggest the possibility that the control mechanisms of reward and attentional systems in the brain are interdependent.