Money and Rock&Roll: Individual differences in reward-guided learning and specific sensitivity to musical reward
SUMMARY
Rewards are constantly influencing our decision-making processes, from the most habitual of our everyday actions to the most important and relevant decisions of our life. Reward seeking behaviour depends on successfully extracting reward information from a large variety of stimuli and events. Through learning, this information allows us to predict the occurrence of rewards in order to adapt our behavior and improve the choices that we will make in the future. Therefore, it seems critical to have a brain system able to compute and assign reward values to those actions and cues that lead to rewarding outcomes in order to improve or maintain successful behaviors. Indeed, these are the main functions attributed to one of the most studied neuronal network in the brain, the reward system. The study of the anatomy and functional properties of this network has increased enormously since the first evidences of its existence, fifty years ago. The present thesis aims to contribute and to open new lines of research in this promising field by addressing questions that, so far, are still unclear.
For instance, the majority of previous research has been mainly focused on describing the brain regions involved in reward processing and the specific computations underlying each of these structures. However, the study of the brain oscillatory activities involved in the coordination of the diversity of regions and networks engaged with reward processes is still in its early stages. On that sense, one of the main goals of this thesis is to characterize the functional role of certain oscillatory patterns previously related to reward processing and learning. In three different studies we have shown that while mid-frontal theta oscillations are critical to integrate information from motivational and attentional networks within the medial Prefrontal Cortex, beta oscillations reflect the interplay between attention, memory and reward-related structures.
On the other hand, most of the studies have used simple tasks, in which individuals might reach rewards or punishments with one single action. However, it is not clear to what extent the same mechanisms can be also applied in tasks that requires pattern of behavior more complexes and to what extent, in such scenarios, individuals may rely on other non-rewarding information to drive behavior. Thus, the second main goal of this thesis is to study reward-guided learning processes in more complex scenarios. In the fourth study of this dissertation we have shown that, in such contexts, participant’s choices may be influenced not only by the amount of rewards obtained but also by the occurrence of non-rewarding events that indicate the accomplishment of sub-goals. In particular, striatal sensitivity to these events predicted participants’ bias, highlighting the relevance of this region in complex learning problems and decision-making.
Finally, a third aspect that is usually ignored in the literature is how the brain process and discriminate different reward types. Although the reward circuit is a common pathway across all kind of rewards, individuals clearly differ in their sensitivity to the diversity of reward-types. On that sense, the third goal of this dissertation is to identify individuals with specific deficits in processing a particular rewarding stimulus but not others, in order to study specific reward responses. In study 5 and 6, we have identified a group of individuals with low sensitivity to musical reward but with average sensitivity to other kind of rewards and no perceptual difficiulties. The identification of specific-musical anhedonics is an important first step for the understanding of the selective recruitment of the reward system.
Thus, this dissertation brings together different lines of research trying to integrate topics that are otherwise treated as independent. Taken together, the research presented here provides a broad view of reward procesess: from sensation and motivation to decision-making and learning. In particular, the results presented and discussed strengthen the idea that sensory information may enter the reward circuit through specific routes of entrances. Moreover, this circuit may be engaged not only by rewards but also by non-rewarding events relevant for driving learning in complex scenarios. Finally, the value computations performed by the circuit might then impact on cognition and memory through several interactive routes orchestrated under particular rhythms leading to an adaptive behavior. I believe that the findings and ideas provided in this dissertation might contribute to the ongoing research in this field: validating previous evidences and offering new insight for further research.