Category Archives: Earworms

A brain basis for musical hallucinations

 

lucerne

Hello Dear Reader,

I hope your January is going well. We are experiencing an unusually mild winter here in Switzerland so far. Very little snow has reached Luzern, which stands 400m above sea level. But at least I can now see snow on the mountains (including the stunning Mount Pilatus) from my office window, and very lovely it looks too.

Today I have been reading a new article (in press) in Cortex which claims to have identified a brain basis or at least a brain based explanation for musical hallucinations (MH). My interest was peaked – perhaps this might give a first clue about a brain basis for earworms?

Earworms (tunes that get stuck in your head) and MHs (complex musical perceptions with no external source) are not the same thing though they have a number of common features, both being musical and related to mental imagery.

The main difference as far as I am aware is one of conscious inference regarding the likely source of the sound: One is clearly recognizable as a memory (earworm) whereas one could easily be mistaken for the experience of real music listening (an hallucination).

A few weeks ago I write a blog about the difference between MHs and tinnitus. This blog was based on a new paper (Vanneste et al., 2013) that compared the resting brain state activity (using EEG) of people who experienced regular tinnitus (a sensation of ringing in the ears) or MH to that of spontaneous activity.

Neuron_in_tissue_cultureThe result of this paper was a theory that abnormal firing of neurons in some bandwidths (alpha, gamma-theta) in the lower centres of the brain was associated with tinnitus.

By contrast, MHs were associated with abnormal neuronal firings in the higher centres of the brain, those associated with memory and language processing.

 

This idea of ‘hierarchical levels’ of abnormal neuronal patterns in the musical brain pathway was a nice intuitively sensible conclusion. It was also interesting to see some brain basis for these conditions as opposed to the more common idea of blaming everything on the inner ear.

The new paper by Sukhbinder Kumar and colleagues takes a case study approach instead. The team looked at the experiences and brain activity of one 62 year old keen amateur musician who had absolute pitch. This lady had experienced MH 15 months after the onset of acute hearing loss, approximately a year and a half before she took part in the study.

When she first started experiencing MH the lady assumed that the sounds came from an external source – by my definition that qualifies them as an hallucination rather than earworms, even though she now knows that these musical sounds are not real (they appear to always be the same few bars of recognizable melodies).

NIMH_MEGThe authors used a clever technique to assess her MHs as they happened in an MEG scanner.

Her MHs could be suppressed by playing short excerpts of music by Bach (like a mask) so the authors compared her brain activity across time in a single scanning session as she moved in and out of a state where she experienced MHs.

 

A beamforming analysis was then performed on the brain data to isolate patterns in oscillatory activity during MH across five frequency bands: 1-4Hz (delta), 5-14Hz (theta/alpha), 14-30Hz (beta), 30-60Hz (gamma) and 70-140Hz (high gamma)

Results: Significant power changes during high periods of MH were observed in the theta/alpha, beta and gamma bands but not in delta or high gamma.

None of these changes were localised to the right hemisphere and all changes referred to increases (rather than decreases) in oscillatory power.

Area 1 of activity was the orbitofrontal cortex (theta/alpha activity). This area has been associated with responses to unpleasant music and imagery. It is perhaps not surprising to see this activity therefore, since the lady was often bothered by her MH. 

Area 2 of activity was the motor cortex (beta activity) which the authors link to the well established activation of motor areas in response to musical imagery, particularly in musicians.

Area 1 of activity was the secondary auditory cortex (aSTG – gamma activity). This area is involved in melody perception.

Traffic_light_greenHow do these results compare to the previous paper? This paper and that of Vanneste et al. (2013) show an increase in gamma in relatively ‘lower’ brain areas (secondary sensory cortices – green) and an increase in alpha and beta power in ‘higher’ brain centres during MH (motor cortex – orange ), which fits with a hierarchical theory of MH.  The present paper takes this hierarchy idea to propose a new model for the brain basis of MH. Their theory presupposes only the presence of hearing loss.

Crucial to this model is the existence of a top down predictor system that we build through a lifetime of musical listening. This system of ‘priors’ sends predictions back through the musical perception pathway in response to sensory stimulation in the level below. Ascending (upward going) information about the music being heard then consists only of any information on prediction error so that higher level expectations can be modified.

It is a Bayesian optimised prediction system for music.

Top down musical prediction from priors – bottom up prediction errors

When someone loses their hearing the brain responds by lowering the sensory precision of the lower sensory centres of the brain (in this case the auditory cortex). That leaves the next level of the hierarchy increasingly sending through prediction error messages to the higher systems, unchecked. And the higher centres reciprocate with backward prediction messages, creating a loop that leaves out the lower level.

In theory this leaves a cycle of communication between the brain areas that drive basic melody perception and imagery (and memory) without the strong input of the lower sensory systems to feedback a prediction error based on what is actually being heard. This leads to a MH.

musicWhy music? As compared to speech or images, music is more predictable and repetitive. It is also rapid and temporal meaning there is more pressure to alleviate strain on the sensory systems, to support them with predictions from higher centres. These combined characteristics mean music is more subject to the activity of priors; music’s own recursive cyclic characteristic is what lends it to be the basis for hallucinations

…and perhaps is why earworms tend to be musical too.

What does this tell us about earworms, and what is missing? If we accept the hierarchical prediction model of the musical pathway then we might presume that any spontaneous activation of the system (for example, in memory) might trigger reciprocal communication within part of this loop. A tune therefore might get stuck in our mind when we have an earworm in the same manner as a MH.

What we don’t know is: a) how this activity is triggered in the first place in either MH or earworms; b) why the loop goes on and on; and c) what part of the model makes the difference between a MH and an earworm

It might explain however, why listening to music often helps people deal with earworms (as you can read about in my upcoming PLOS ONE paper on earworm cures!), as the ascending musical input of prediction errors in this case would break the cycle of internal musical imagery.  

What does it NOT tell us about MH? This paper provides an explanation for MH as a result of hearing loss – it does not provide an explanation for MH that are experienced by people with a psychosis or as a result of a focal brain lesion. Why do people experience MH when there is, on the face of it, nothing wrong with the lower hierarchy (the sensory systems)?

Conclusions

This is a really nice paper that stimulated great conversation in my office about the nature of perception and mental imagery, both in the musical world and beyond!

I think these Bayesian network ideas are here to stay so I advise you to have a go at reading this paper and to think about how the brain’s way of dealing with rapid, temporal sensory input (i.e. relying on top-down predictions) might influence what we experience as part of consciousness.

Article: Kumar, S., et al., A brain basis for musical hallucinations, Cortex (2014)

What is the difference between musical hallucinations and tinnitus?

Before I left for Switzerland at the start of this month I printed off a few articles that I had hoped to read for background research, and also to provide you, dear reader, with some of the latest findings in music psychology.

995998_10151888557415991_774341405_nWell, this hope has been slightly delayed. It has been a long few weeks here in Luzern as I have been balancing immigration admin, with the freedom that comes with being able to write up articles that have been sitting on my desk for months. I love being able to finally sit down to write up these interesting research projects. I have also been enjoying the planning and teaching of my new Music Psychology course, here at the Hochschule Luzern – Musik.

All this is why it has taken me a little while to get to my exciting pile of new papers. Here I am finally though, reading the first one.

This new paper tackles the question of whether musical hallucinations and tinnitus are similar phenomena. This is of interest to me as these two kinds of ‘auditory phantom’ may also be on a continuum with earworms, tunes that get stuck in our head (a keen research interest for me).

What is tinnitus? Tinnitus is when a person hears a simple tone or buzzing noise that is not coming from the environment. The term comes from the Latin word tinnire meaning “to ring”. Many of us will have experienced temporary tinnitus, perhaps after visiting a concert or nightclub, or exposure to other loud and prolonged sounds.

I Hear MusicWhat are musical hallucinations (MH)? MH are complex false sound perceptions meaning that a fully conscious person might report hearing whole songs or other musical arrangements, all of which have no external source. Musical hallucinations are more common in older adults, especially those who are suffering from a degree of hearing loss. MH can also arise from brain lesions at any point in the auditory pathway, from the lower centres (e.g. brainstem) right through to the auditory cortices.

What do these two things have in common? Both MH and tinnitus represent the perception of a sound stimulus in the absence of any physical stimulation from the environment – people are hearing things that are not ‘there’.

In both cases people can often not realize at first that these sounds are not ‘real’ – they may go searching for the source of the sound (a hidden radio or a buzzing piece of electrical hardware) eventually to realize that the sound is being generated internally rather than externally. Because people can eventually recognize the internal source of the sound MH and tinnitus are not, by themselves, a form of “psychosis”, which is defined as a loss of contact with reality.

The new paper I am reading hypothesised that MH and tinnitus might share a common source in the brain that differs only in its complexity, with tinnitus producing simple sound and MH producing complex music.

Neuron_in_tissue_cultureThe model: The model the authors chose was ‘Thalamocortical dysrhythmia’ (TCD), a suitably complex term (go on, say it three times fast, I dare you!). TCD is a theory that suggests the neurons in the thalamus can start to behave in an odd way, either diminishing their excitation or increasing their inhibition. This haywire activity at the neural level, known as ‘deafferentation’, can trigger both positive and negative symptoms in neurological conditions.

How might TCD cause tinnitus and MH? According to the new paper, tinnitus may be caused by abnormal, spontaneous and constant gamma band activity as a consequence of a particular pattern of deafferentation in the thalamic nuclei. This can be combined with an increase in theta activity, and perhaps also a decrease in alpha activity. This combination of abnormal firing patterns is known as the “edge effect”

The difference that turns tinnitus into MH might be found in brain areas like Broca’s , regions of the brain that handles both language and music stimuli. Perhaps a low level “edge effect” combined with higher brain centre abnormal activity turn simple tinnitus into MH?

UnbenanntThe test: The new study used EEG to compare resting state brain activity between healthy controls, and people with either tinnitus or MH. The hypothesis was that both simple (tinnitus) and complex (MH) auditory phantoms will share a common neural signature of increased gamma-theta activity in combination with decreased alpha activity, as in TCD. Furthermore, only MH should involve traces of abnormal activity in higher music and language areas

The authors compared 10 people in each group (control, tinnitus and MH). The two clinical groups had both had their symptoms for at least a year and had comparable hearing loss (not severe).

The results: People with MH or tinnitus both showed increased gamma-theta activity within their auditory cortex. This finding alone suggests that TCD might underlie both simple (tinnitus) and complex (MH) auditory phantoms.

Interestingly both groups also showed suppressed beta band activity in their visual cortices. The reason for this is not yet known, but it has been reported in previous studies.

There was also increased beta activity in areas such as the dorsal anterior cingulate cortex and anterior insula during both tinnitus and MH. This activity might be related to the brain ‘filling in’ missing auditory information and brining the phantom auditory percepts to awareness.

Gray726_inferior_frontal_gyrusWhat is unique in MH? People with MH showed more alpha activity, which we can explain as the increased memory load of dealing with complex percept as opposed to a simple buzz or ring of tinnitus. People with MH also showed increased resting state activity in right inferior frontal regions, areas associated with retrieval of familiar musical imagery from memory, and areas associated with language and music processing.

Conclusions: The TCD model may prove to be a suitable explanation for both musical hallucinations and tinnitus, since both were seen in the new study to feature the relevant abnormal patterns of neural activity. The future will tell us if similar patterns are seen in prolific earworm sufferers.

Complex phantoms (MH) in addition are associated with additional patterns of abnormal neural activity in various bands across areas of the brain that are associated with memory and musical imagery, but also language processing.

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Sven Vanneste, Jae-Jin Song, Dirk De Ridder (2013) Tinnitus and musical hallucinosis: The same but more. NeuroImage 05/2013; DOI:10.1016/j.neuroimage.2013.05.107. Find and request a copy of the paper here.