Expectation and surprise: The key elements that make pop classics irresistible

In a study, researchers analyzed 80,000 chords in 745 classic US billboard pop songs and found that musical pleasure comes from the right combination of uncertainty and surprise.

The mix of these two patterns makes the difference

Why do many people find songs like James Taylor's "Country Roads", UB40's "Red, Red Wine" or The Beatles' "Ob-La-Di, Ob-La-Da" so irresistibly pleasant? In a study, researchers analyzed 80,000 chords in 745 classic US billboard pop songs and found that musical pleasure comes from the right combination of uncertainty and surprise.

"It is fascinating that people enjoy a piece of music only by the way the chords in music are arranged over time," said Vincent Cheung of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany. "Songs that we find pleasant are probably those that strike a good balance between our knowledge of what will happen next and the surprise of something we didn't expect," the neuroscientist explained. "Understanding how music activates our pleasure system in the brain could explain why we often feel better when listening to music, even when we are melancholic.

Cheung and his colleague Stefan Koelsch used a machine learning model to mathematically quantify uncertainty and surprise in 80,000 chords of US Billboard pop songs. The researchers freed the songs from elements such as text and melody and retained only the chord sequences. Thus they wanted to exclude existing associations of the listeners to the songs and make sure that the chart hits were not immediately recognizable.

Uncertainty of listeners' predictions has not been taken into account so far

The researchers were finally able to prove it: individuals were relatively sure which chords to expect next, but they found it pleasant when they were surprised instead, i.e. their expectations were not met. On the other hand, if individuals were unsure of what to expect next, they found it pleasant when consecutive chords were not surprising.

"Although composers have known it intuitively for centuries, the underlying process of expectation in music was still unknown," Koelsch said. "In the past, most studies have looked only at the effects of surprise on musical pleasure, not at the uncertainty of listeners' predictions.

Activity in the nucleus accumbens reflects only the uncertainty of the listeners

Using brain images from functional magnetic resonance imaging (fMRI), the scientists found that the experience of musical enjoyment is reflected in three brain regions: the amygdala, the hippocampus, and the auditory cortex. In contrast, and completely surprising for the researchers, the activity in the nucleus accumbens only reflected the uncertainty of the listeners. Until now, it was thought that this part of the brain also plays a role in the processing of musical pleasure.

"In summary," the researchers wrote, "we show that musical pleasure depends on the dynamic interplay between prospective and retrospective states of expectation. Our basic human predictive capacity is, therefore, an important mechanism by which abstract sound sequences acquire affective meaning and become a universal cultural phenomenon that we call 'music'".

Combined role of uncertainty and surprise should be given more attention

Based on their findings, the study authors encourage future brain research to bring more focus to the combined roles of uncertainty and surprise - and thus, for example, to find out why other art forms such as dance and film have such a high value for people. The results could also be used to improve artificial algorithms that generate music, to help composers write music, or to predict musical trends.

The next step for the neuroscientists themselves is to investigate how information flows through different parts of the brain when listening to music over time. They want to know why and how it happens that people who listen to music sometimes get goosebumps. Cheung said: "We think that there is great potential in combining computer modeling and brain imaging to understand not only why we enjoy music, but also what it means to be human.”

Cheung VKM et al., Uncertainty and Surprise Jointly Predict Musical Pleasure and Amygdala, Hippocampus, and Auditory Cortex Activity; Current Biology 2019; DOI:10.1016/j.cub.2019.09.067