How music helps fight the effects of Parkinson's disease

The beating of our heart, the flow of air through our lungs, the prosody of our voice or the movement of our legs... Our body is a sea of different rhythms. None of the biological systems that generate them is isolated: each interacts with its environment, made up of other systems inside or outside our body that have their own rhythmicity.

Loïc Damm, University of MontpellierBenoît Bardy, University of Montpellier and Valérie Cochen de Cock, University of Montpellier

Movement is a fine example of how rhythms interact. The simple act of walking, step by step, is indeed a complex construction! When this construction breaks down, movement suffers. Hence the question: can we restore rhythm to those who have lost it? Yes, suggests some research. Here's a brief explanation, starting with the basics...

While our brain is at the helm, a whole range of nervous structures govern the associated cycles: networks in the spinal cord provide the alternating muscular activations necessary for its genesis, but it's the higher brain centers that provide the plasticity, since it's at their level that movement initiation is planned, or the conditions for its proper execution (obstacle avoidance, etc.) are taken into account.

The basic program originating in the networks of the spinal cord is thus remodeled according to the demands of the environment, transcribed by our senses... What neuroscientists call "perception-action coupling" takes place: because it controls our muscles and integrates auditory or visual information, the nervous system is capable of coupling our senses to our behavior.

This is what happens when we play music in a group, where the temporal coordination of our gestures with those of our partners enables us to be in unison. This synchronization is made possible by the adjustment between auditory/perceived and motor/executed rhythms. This means that structures more dedicated to perception and those more dedicated to movement see their links strengthened, forming a functional network in the brain.

In other words, the brain structures that make us move are also those that make us perceive. When listening to pieces of music, which combine structured sequences of durations, timbres and accents, the perception of pulsation is the most regularly recurring psychological event.

When illness derails the machine

Pathologies can interfere with the production of these rhythms. This is the case with Parkinson's disease, for which difficulties in moving around are the main handicap reported.

Patients are subject to "gait freeze", i.e. difficulty in initiating and progressing when approaching an obstacle or bend. These two major movement sequences are affected by the progressive loss of neurons secreting "dopamine" - a neurotransmitter, a molecule that transmits information between nerve cells.

A so-called deep brain structure, buried beneath the cerebral hemispheres, the basal ganglia, is particularly affected. These ganglia manage the transition from one stage of a movement to the next: altering their functioning will therefore affect the entire production of rhythmic movements, by disrupting the cerebral rhythms required to trigger the sub-movements that make up an action.

A choppy gait is symptomatic of this difficulty in switching from one sub-movement to another. If a patient with irregular gait can continue to pedal more fluidly, it's because pedaling is less dependent on sequential processing of sensory information.

The gait cycle requires the premotor cortex to take into account a wide range of sensory information, both in terms of environmental constraints and the correct execution of the movement in progress: this process is known as integration. A good connection between the cortex and the basal ganglia enables the gait to be adapted to the specificities of the environment - a bend to anticipate, a staircase to negotiate, a street to cross...

The loss of dopamine neurons inherent in Parkinson's disease prevents these connections from being established (known as circuitopathy). This results in a broad spectrum of motor effects, from locomotion to speech.

The effects of music

Yet this deficit can be overcome with a simple strategy: simply by taking advantage of the brain's appetite for "relevant" rhythms, those with which we can synchronize our movements.

The use of an external clock to provide regular cues, such as periodic auditory stimuli, helps to compensate for difficulties in initiating and maintaining movement by restoring a temporal structure to actions. This strategy is known as " cueing".

Patients benefit from this visual, tactile and auditory cueing - the latter making it easier to discriminate the rhythms sent. This is evidenced by an increase in the cadence and length of steps, as well as correction of gait asymmetries. Patients walk faster, and their increased stability reduces the risk of falling. This improvement reflects the better coupling between auditory flow and the locomotor apparatus at brain level. These benefits extend beyond the musical walking sessions.

There are two possible explanations (which are not exclusive) for these improvements:

• residual activation of the basal ganglia,
• the development of compensatory mechanisms involving the cortex and cerebellum. Hyperactivation of the cerebellum has been reported in patients during sensory-motor coordination tasks.

A major element was highlighted: the precision of rhythm perception determines the strength of the coupling between locomotion and music.

Tests have been developed to assess our perceptive abilities on the one hand, and our ability to synchronize our movements with music on the other. For example, do we notice the time lag between an out-of-sync metronome and the beat of the music? Are we capable of keeping time with pieces of more or less obvious rhythmicity?

The accuracy of beat perception and regularity is representative of coordination skills, and can be compared with established standards. This makes it possible to quantify impairment, and can sometimes serve as anaid to diagnosis.

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Rehabilitation possible

There is an apparent contradiction between the beneficial influence of indiçage and the deterioration in patient perception.

Concurrent assessment of perceptual and gait abilities under the influence of auditory stimulation has clarified this point. The patients who benefit most from indiçage are those who have preserved their perceptual abilities. This reinforces the hypothesis of the primacy of the strength of the audio-motor coupling in predicting the benefits of indiçage.

However, the consequences of impaired perception are not inevitable. Re-training is possible through serious games in which the patient relearns to synchronize with music, through dance, which is a sensorimotor synchronization activity par excellence, and so on.

If walking is improved by auditory cues delivered at the right tempo, the interactivity of these stimuli is also an essential factor to consider to improve coupling strength.

We have shown that real-time control of the relationship between musical pulses and the patient's steps, through continuous adaptation of the musical tempo, guarantees ideal audio-motor coupling. Combined with the positive effects of music, both neurochemical through the release of pleasure hormones, and psychological through the sense of escape it provides, the effects of stimulation on walking are immediate. The benefits of indiçage are further enhanced.

One of the current challenges is to assess the long-term effects of such an approach.

Medium- and long-term prospects

Full perception of music requires motor structures: don't we need to move our whole body to keep time with a difficult piece? The overlap between the structures in our brain that enable us to perceive, and those that move us, opens up a therapeutic opportunity.

The re-education of movement through music strengthens the links between rhythmic auditory perception and motor behavior. Music interferes with the brain's motor networks and can compensate for certain deficits created by Parkinson's disease. This approach can help improve patients' quality of life and reduce their dependence on medication. It is therefore a powerful rehabilitation tool that complements pharmacological therapy.

Other pathologies involving motor deficits are also concerned. Multiple sclerosis, stroke and type II diabetes are currently being studied by our team.

Loïc Damm, Postdoctoral Researcher, University of MontpellierBenoît Bardy, Professor of Movement Sciences, founder of the EuroMov center, member of the Institut Universitaire de France (IUF), University of Montpellier and Valérie Cochen de Cock, Doctor of Neurology, HDR researcher with the EuroMov Digital Health in Motion unit, Université de Montpellier - IMT Mines Ales, University of Montpellier

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