Hedonic responses to music are the result of connectivity between structures involved in auditory perception as a predictive process, & those involved in the brain's dopaminergic reward system

Musical anhedonia and rewards of music listening: current advances and a proposed model. Amy M. Belfi, Psyche Loui. Annals of the New York Academy of Sciences, September 23 2019. https://doi.org/10.1111/nyas.14241

Abstract: Music frequently elicits intense emotional responses, a phenomenon that has been scrutinized from multiple disciplines that span the sciences and arts. While most people enjoy music and find it rewarding, there is substantial individual variability in the experience and degree of music‐induced reward. Here, we review current work on the neural substrates of hedonic responses to music. In particular, we focus the present review on specific musical anhedonia, a selective lack of pleasure from music. Based on evidence from neuroimaging, neuropsychology, and brain stimulation studies, we derive a neuroanatomical model of the experience of pleasure during music listening. Our model posits that hedonic responses to music are the result of connectivity between structures involved in auditory perception as a predictive process, and those involved in the brain's dopaminergic reward system. We conclude with open questions and implications of this model for future research on why humans appreciate music.

Introduction

The capacity to perceive, produce, and appreciate music, together termed musicality,1 has been a growing topic of interest in the past 20 years of cognitive neuroscience. While most cognitive neuroscience studies on musicality focus on music perception and production skills, there has been a recent explosion of interest in the appreciation of music.2, 3 Multiple research programs in the cognitive neuroscience of music have involved comparing participants with different types and levels of musical training.4-7 However, to cognitive neuroscientists who are not particularly concerned with music, these studies may appear to be highly specialized and of limited interest as they seem to focus on a special population—highly trained musicians. In contrast, studies on the appreciation of music can be thought of as more general and inclusive, encompassing the vast majority of humans regardless of formal musical training.

Humans show knowledge of fundamental musical building blocks, such as rhythm and beat, from as early as 1 day old,8 and as shown from the success of the multibillion‐dollar music industry, humans around the world enjoy music. One of the most frequently reported reasons for listening to music is the overwhelming influence it has on feelings and emotions.9 Music has been deemed an ultimate group bonding activity;10, 11 this is supported by structural features of melody, harmony, and scales that are observed across many cultures,12 as well as the ubiquity of songs that serve social functions, such as lullabies, dance songs, healing songs, and love songs, across cultures.13 Singing and making music together enhance social interactions and group bonding14, 15 and elicit physiological effects that are observable from infancy.16 Even in the few cultures where music is not produced in groups, members of these cultures nevertheless enjoy singing for each other,17 suggesting that the capacity for music enjoyment, that is, the rewarding aspects of music, may be intrinsic to humans as a social species. Together, these lines of research suggest that understanding why humans love music may offer a window into how humans interact in a social environment.

The rapid growth of research on musical enjoyment, specifically in cognitive neuroscience, may also be facilitated in part by recent findings on the role of dopamine in coding for prediction and reward. Since the classic observations that stimulating dopaminergic neurons elicits motivated behavior,18 and that dopaminergic neurons signal changes in the predictability of rewards,19 thousands of studies have identified a set of regions within the human brain that are especially sensitive to reward. These regions center around the midbrain (the ventral tegmental area and substantia nigra (SN)), the dorsal and ventral striatum (VS) (the caudate, putamen, nucleus accumbens (NAcc), and globus pallidus), and the medial prefrontal cortex (mPFC).20 These areas, which we refer to throughout this article as the reward system, are reliably activated during the experience of unconditionally rewarding, evolutionarily salient stimuli, such as food and sex, as well as stimuli that are strongly associated with such rewards, such as money. Findings from the monetary incentive delay task show that cues that predict monetary rewards reliably activate the striatum and mPFC,21 core areas of the reward network. Interestingly, activity in the striatum is also observed when social stimuli (faces) are substituted for monetary rewards,22, 23 suggesting that social and monetary cues tap into “common neural currency” of the reward system.22

The findings that food and sex activate the same reward system can be readily explained as being evolutionarily adaptive: being motivated to seek out these stimuli improves our chances of survival. In contrast, the adaptive value of music—and aesthetic stimuli more generally—is less obvious. Nevertheless, much recent work has shown that music engages the reward system (as reviewed below; see also Ref. 24). While music ranks highly among the pleasures in life,25, 26 recent work has identified a unique condition of people with specific musical anhedonia:27, 28 people who are insensitive to the rewarding aspects of music despite normal hedonic responses to other sensory and aesthetic stimuli, and normal auditory perceptual abilities.29 The existence of this unique population raises many important questions. Some of these questions include:

.    The nature versus nurture of musical reward sensitivity: Does musical anhedonia run in families? When and how did it develop? What, if any, genetic underpinnings might predispose an individual toward musical anhedonia? What is its developmental trajectory?

.    Domain‐specificity versus domain‐generality of reward sensitivity: Are there specific neural pathways for music reward that are separate from general reward? What are the neural pathways through which specific stimuli (such as music) come to have privileged access to the reward system? What endows a certain stimulus with privileged access to the reward system?

.    Psychological associations and clinical comorbidity: What are the associations between musical anhedonia and psychological traits, both in the normal range (e.g., big five personality traits) and clinical populations? What is the comorbidity between musical anhedonia and personality, mood, and communication disorders?

.    The evolution of music: To what extent do nonhuman animals also show reward sensitivity to music? Have there been people with musical anhedonia for as long as there has been music? By extension, if people with musical anhedonia have survived for generations with no apparent disadvantage alongside the rest of the population who have normal reward sensitivity to music, then the lack of reward sensitivity to music seems not to affect their survival. If this is the case, then why do we seek out music?

Here, we review the recent cognitive neuroscience evidence for musical engagement of the reward system, as well as an extreme end of the spectrum of individual differences in sensitivity to music reward in specific musical anhedonia. Based on our review of the literature, we propose a model that accounts for the nature of the auditory access to the human reward system, and its disruption in musical anhedonia.