Digging Out the “Earworms:” Involuntary Musical Imagery Correlates with Cortical Structure

You don’t know you know this song, but you definitely know this song: “Hey Mickey you’re so fine, you’re so fine you…” Did the end of the lyric materialize in your mind, complete with musical accompaniment? Your memory of Toni Basil’s “Mickey” is so ingrained, it will probably continue to annoy for several hours after you finish reading this article. Sorry. However, the example illustrates “stuck song syndrome” or more formally, involuntary musical imagery (INMI), a universal phenomenon of having music looping in one’s head. How do musical tunes affix to the architecture of the brain? Is the structure of some brains “stickier” than others?

INMI is typically triggered by recent musical exposure, low attention states, and associations between memories. Individual differences arise in personality, where traits such as obsessive-compulsive tendencies and neuroticism are associated with more frequent INMI. Yet, due to the elusive nature of INMI, functional neuroimaging has proved difficult. Besides the obvious challenge of obtaining a brain image at the exact moment INMI is occurring, the MRI scanner produces rhythmic, loud sounds that might interfere and perhaps even drown out INMI experiences. Researchers at University of London speculated neural correlates of this phenomenon are important to elucidate given the possibility INMI influences moods and emotions akin to instances of actual music listening. Is it possible that discrete aspects of INMI experience co-vary with regional differences in brain structure?

A new measure, the Involuntary Musical Imagery Scale (IMIS) explores facets of the INMI experience, such as how often a certain type of music was heard as well as sensorimotor and even self-reflective aspects of this experience. The London group used the IMIS test in concert with gray matter (GM) volume analysis to investigate brain region differences associated with INMI. The results showed correlations of the frequency of INMI episodes with cortical thickness in the primary auditory cortex: right Heschl’s gyrus (HG) and right inferior frontal gyrus (IFG).

Heschl’s gyrus has been strongly implicated in both auditory perception and voluntary musical imagery, while the inferior frontal gyrus is believed to have a role in pitch memory, auditory perception and imagery. It can be surmised that this fronto-temporal network, which is implicated in the sensation of actually hearing music, may also be involved in the weaker sensation of imagining music. Furthermore, the IFG may play a dual role in both facilitating and suppressing INMI episodes, owing to the fact that it is involved in inhibitory processes in general.

INMI is neurologically similar to self-generated thoughts (such as mind wandering or day dreaming), which previous literature has shown to germinate from the default mode network (DMN). The DMN is active when we are not focusing on a task or are resting. The form and content of self-generated thoughts are reflected in the spontaneous fluctuations during resting state in the DMN areas that include medial prefrontal cortex, posterior cingulate cortex, parahippocampal cortex (PHC), and ventral anterior cingulate cortex (ACC). Indeed, the tendency to imagine music is phenomenologically and mechanistically similar to daydreaming. A core component of the DMN, called the ACC, was found to have reduced cortical thickness and was associated with both greater frequency and longer length of INMI episodes. While the role of the ACC is still undefined, it has been implicated in task-unrelated thoughts, which indicates INMI shares neural mechanisms with the DMN.

How do we feel when we hear “Mickey?” Irritated, I’m sure. The emotional role of INMI was investigated by evaluating the extent to which participants rated earworm experiences as either helpful or disturbing. These answers were correlated with the gray matter (GM) volume of orbitofrontal cortex (OFC), an brain region responsible for emotional processing of external sensory stimuli. Interestingly, it also regulates and inhibits internal stimuli, such as internally evoked music emotions. GM volume was greater in the OFC of subjects who reported higher disturbances in response to INMI episodes. The tendency to be bothered by INMI may be linked to the inability of the OFC to inhibit negative emotions associated with the experience of INMI.

Finally, why do we remember “Mickey” so well? Increased gray matter (GM) volume was shown in the parahippocampal cortex (PHC), an area activated both in the voluntary and involuntary recall of episodic memories. In particular, PHC GM volume was increased in subjects with a higher tendency to experience positive emotions related to music. Greater GM volume in the PHC might facilitate retrieval of memories or emotions related to the INMI, while less GM volume may lead to reduced salience of retrieved memories, preventing interference of the memory with the task at hand. In summary, more gray matter in this region suggests a tendency to engage in non-task related thoughts: daydreaming, or in this case, dayhearing, particularly if music elicits highly pleasant emotions.

If you are wondering why some songs wriggle their way into your ear canal and become more easily attached than others, it involves several factors.

1. Repetitiveness: Grey matter devoted to memory in the PHC might mean more “looping” of the same maddening hook.
2. Song Structure: If the song is predictable and enjoyable (has a mild incongruity), a neural path already exists for it and the brain will loop it ad infinitum.
3. Mood: When the brain is taxed, the orbitofrontal cortex, which governs which stimuli need to be inhibited, also slacks off, which might allow “Mickey” to stick indefinitely.

Since earworms are only pieces of songs, “deworming” usually involves listening to the entire song and completing the loop.

The results of this unique study correlate several pieces of “stuck song syndrome” with cortical structure variability, suggesting that one’s degree of hearing and reacting with exasperation to “Mickey” may depend on the anatomical structure of one’s brain. Check out SciFri’s episode on this topic and a list of songs scientifically proven to worm their way in.

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Written by Teodora Stoica.

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References:

Nicolas Farrugia, Kelly Jakubowski, Rhodri Cusack, & Lauren Stewart, Tunes stuck in your brain: The frequency and affective evaluation of involuntary musical imagery correlate with cortical structure, Consciousness and Cognition, Volume 35, September 2015, Pages 66-77, ISSN 1053-8100, http://dx.doi.org/10.1016/j.concog.2015.04.020.

Images made by Jooyeun Lee.