Introduction

Music has psychological and physiological influence on human body. This article aims to shine a light on how emotions showcase themselves in psychophysiological and neurological ways - transforming your favourite tune into a tool for changing the mental state. According to Juslin and Vastfjall, emotions are defined as "Relatively intense affective responses that usually involve a number of sub-components - subjective feeling, physiological arousal, expression, action tendency, and regulation - which are more or less 'synchronized'. Emotions focus on specific objects, and last minutes to a few hours". There are two contemporary and competing theories, about the human experience of emotion in relation to music. Cognitivists hold that emotions are perceived from music, but not necessarily felt, while emotivists recognize that music induces true emotional experiences. The general theory is that physiological and neurological methods would not measure a demonstrated reaction to music if emotional expression from music is perceived but not felt. This paper explores this theory by looking at an excerpt on psycho-physiological measures by Donald A. Hodges from Handbook of Music and Emotion: Theory, Research, Applications and Brain, music and musicality: Inferences from neuroimaging by Robert Turner and Andreas A. Ioannides. We’ll look at the current state of psychophysiological and neuroimaging research, the complexities and limitations in these lines of research, and where further research is needed.

Psycho-physiological Measures of Emotion

Psychophysiological Response

A psychophysiological response is a physiological change, such as blood pressure, heart rate, respiratory rate, body movements, facial expressions, skin conductivity, and more, in response to human behavior or experiences. These behaviors can include memory, attention, sleep, and emotional responses. All physiological responses originate from the autonomic or somatic nervous systems. We’re more interested in the autonomic nervous system (ANS) which is responsible for keeping a stable internal environment despite stimuli. Homeostasis by the ANS requires constant monitoring to detect changes and respond to handle the changes accordingly. The autonomic nervous system can be further subdivided into:

• Sympathetic nervous system (SNS) – reacts in fight or flight situations. When humans experience a stimulus, the SNS is trying to decide how best to react to each situation. This system is what automatically increases heart rate, muscle tension, and adrenaline in stressful or dangerous situations, to help prepare the body to either run away or fight.

• Parasympathetic nervous system (PNS) - is responsible for recovering from these flight or fight reactions, returning the body to a state of rest, and conserving energy. The PNS slows heart rate, returns digestion to normal levels, reduces muscle tension, and lowers blood pressure, among many other actions.

Previous research into the autonomic and somatic nervous systems has already established a clear link between physiological reactions originating from the nervous system and emotional experiences.

Heart Rate

Heart rate is recorded using an electrocardiogram in the number of beats per minute (bpm). Heart rate has been linked to emotional responses, increased mental activity, and high motor performance (physical activity). 54 studies, ranging from 1906 to 2009, found a link between music and heart rate changes. The majority found that high arousal music increases heart rate among listeners. And low arousal music or sedative music decreases heart rate. Research suggests gender impacts physiological responses. Men with a preference for arousing music had lower resting heart rates, while women had higher resting heart rates. Studies examined the link between performing and heart rate, finding that heart rate was highest for conductors and musicians during performances, especially the most emotional passages. Another 26 studies found no link between music listening and heart rate changes.

Biochemical Processes

This next area of interest, biochemical changes in response to music, shows promise as an area of research but has not been thoroughly investigated yet. Biochemical compounds control all of endocrinology, immunology, metabolism, neurology, and many other bodily systems. Early research indicates biochemical changes are significant when listening to music, but much of the research is still conflicted, so this is a major area of interest. Research finds might help develop music medicine methods that can reduce anxiety, pain, and necessary drug dosages. Blood glucose, interleukin-6 and interleukin-10 (immune response), neutrophils and lymphocytes, and testosterone (in males) decreased in response to music. While cortisol, dopamine, growth hormone, interleukin-1 (immune response), melatonin, norepinephrine, secretory immunoglobulin A, and testosterone (in females) increased when listening to music. Serotonin is the feel-good hormone associated with increased mood and decreased anxiety. Serotonin increased in response to pleasant music \cite{evers2000changes} and decreased in response to unpleasant music. The research is still very conflicted for biochemicals like epinephrine, cortisol, adrenocorticotropic hormone (ACTH), beta-endorphins, and dopamine so further research is needed to tease out the true biochemical changes.

Skin Conductance

Skin conductance is strongly linked with affective changes. For example, under arousing conditions skin conductance increases while resistance decreases. Skin conductance is the level of electrical resistance of the skin. Essentially skin is a better electrical conductor during arousing situations. Skin conductance data is collected by applying a low voltage level to the skin (not felt by the participant) and then recording the electrical resistance changes. 36 studies found significant changes in skin conductance when subjects listened to music, but 6 studies found no link between skin conductance and music.

Respiration

Respiration rate is measured using respiratory inductance plethysmography (RIP). RIP records participants' lung volume using recording bands that are placed around the chest and midsection. Existing research shows that respiration is strongly associated with emotional experiences. 19 studies examining the link between music and respiration, found that respiration increases when listening to music. Research found that following a high-stress task, low-tempo music helps reduce breathing rate. Only 6 studies showed no link between the two.

Muscular Tension

Muscular tension is when muscles are contracted for a period of time. Excessive muscular tension is linked to the stress response. Muscular tension shows a great correlation with music listening. Of the 13 studies examining this relationship, only 2 studies showed no muscular tension changes. Sedative music elicits a faster tension reduction than stimulative music elicits increasing tension. Another component of muscular tension is facial expressions. Craniofacial muscles contract and relax to create facial expressions. Facial expressions can be measured by placing electrodes on the following muscle groups:

• Zygomaticus – the smile muscle, extends from the corner of the mouth across the cheekbone

• Corrugator – muscles of eyebrow associated with frowning

• Orbicularis oculi – muscles under the eye

The electrodes measure the electrical activity of the underlying muscle tissues. Research shows that the zygomaticus showed increased EMG readings during positive music, while the corrugator showed increased readings during negative music. Arousing music with a positive affect yielded the zygomaticus's most significant smiling activity.

Skin Temperature

In previous studies, skin temperature has been linked to emotional responses. But this link is not as obvious in research that has looked at skin temperature and music. While 15 studies have shown skin temperature changes elicited by music listening, there is almost no consistency among the data. There is no definitive link between blood flow in skin tissue and music.

Chills

Chills or frisson is a psychophysiological response usually associated with auditory or visual stimuli that elicit a pleasant and positive affective state. Chills can include physiological responses like tingling, goosebumps, shivering, prickly feelings along the back of the neck, crying, and a lump in the throat. The vast majority of participants, 75-96% that were interviewed affirmed they had experienced chills when listening to music and that it’s a recurring experience. Research showed that chills were also associated with other physiological responses including increased heart rate and skin conductance. A sudden unexpected direction in the composition of the music had the best chance to elicit chills.

Body Movement

It’s well known that humans often respond to music by dancing, nodding their head, tapping their feet, or swaying. The movement seems to be especially encouraged by rhythmic music. Researchers believe that the link is related to how the brain connects auditory temporal information to motor movement.

Summary of Psychophysiological Findings

Considering the large number of empirical studies mentioned above, that measure psychophysiological changes in participants, there seems to be a clear indication that music can induce real emotions. The physiological reactions observed in response to music closely match those of emotional responses seen in other studies. At present, more researchers support this emotivist position than the cognitivist opinion.

Neurological Measures

All human experiences can be traced back to the brain. When attempting to understand how humans experience music and if it elicits a true emotional response, it is necessary to look at neurological activity to get the whole picture. Neuroimaging can help reveal if there is a biological connection between emotions and listening to music, or if our responses are more culturally driven. Emotional responses are more likely to be felt than perceived if research shows that the human brain has musical competency. Neuroimaging techniques like PET, MEG, and fMRI can reveal where and when the brain has increased activity. Current research hopes to understand if any aspects of music are ‘hard-wired’ into the brain and if so what areas of the brain perceive musical components.

Blood Flow Response

One of the methods researchers can use to study neuronal activity is cerebral blood flow. Cerebral blood flow can be measured using PET and fMRI. Both measurement methods have their own limitations and complexities, such as time and spatial constraints. Yet this area of research has the potential to reveal much about the general areas of the brain that are active when listening to music.

PET

Positron Emission Tomography or PET is a neuroimaging technique that can measure physiological activity in the brain by tracing a radioactive isotope. Isotope oxygen-15 is injected into a vein, which then decays into nitrogen-14. A positron is emitted from the decay, which can travel roughly 50mm. Mathematical algorithms map these positrons to determine cerebral blood flow. During the PET scan participants are presented with musical compositions to measure where cerebral blood flow is concentrated, indicating metabolic activity. If the same brain regions that are associated with emotion light up when listening to music, this is evidence of an emotional response to music.

PET Limitations and Advantages

The 50mm range for positrons is the greatest limitation for PET scans. Brain maps from PET scanning only have a spatial resolution of 5mm. To get the best results, without surpassing the dosing limitations of a single participant, results must be averaged across multiple participants to create a higher resolution brain map. Another limitation of PET scanning is the temporal limits. Researchers can only play about 2 minutes of music, roughly six verses of a song. This means that positron emission tomography cannot image the effects of music on cerebral blood flow over an extended time period, to see if the effects of music are long-lasting. PET scanning is advantageous for studying the connection between brain activity and music because the process is completely silent. Participants can clearly hear the musical composition without any background noise to clutter the results.

fMRI

fMRI stands for functional magnetic resonance imaging which reveals neural activity by measuring the changes in blood flow and oxygenation. fMRI takes advantage of the different properties different human tissues have. Deoxygenated blood, also known as venous blood, appears on an MRI scan. Any changes in oxygenation of the venous flood is observed in an MRI scan. As neuronal activity increases, the blood becomes more oxygenated and starts to match the surrounding tissues resulting in a rise in MR image intensity.

fMRI Limitations and Advantages

The primary disadvantage of fMRI is the very loud noise generated during the scan, but the gradient coils. The sound has no modulation and is wholly repetitive, but can impede the experience of music listening. Researchers continue to look for improved noise cancellation methods to mitigate the noise. fMRI also has a low temporal resolution when compared to some other imaging techniques, down to half a second. Brain processing is significantly faster than half a second meaning information is missing. fMRI is good for studying the link between music and the brain because of the wide availability of MRI scanners and the lack of dosing. Participants can submit to repeated scans without limit. The spatial resolution is also better than PET, down to 3 mm or less.

Electrophysiological Methods

Electrophysiology studies the electrical activity of neurons throughout the brain. Electrical activity in the brain can be measured using EEG or MEG. Let’s look at the further breakdown of these two methods.

Electroencephalography (EEG)

An electroencephalogram measures electrical activity in the brain by attaching a net of electrodes to the scalp. EEGs are commonly used to diagnose epilepsy and other seizures or to diagnose brain death following a traumatic brain injury.EEGs are noninvasive and can be repeated as many times as necessary. EEG detectors have a high temporal resolution, down to a fraction of a millisecond meaning they can detect incremental changes in neuronal activity.

Magnetoencephalography (MEG)

MEG is a relatively new method for recording neuronal activity. A magnetoencephalogram machine is shaped like a helmet that fits around the head of the patient or study participant. Within the helmet, tiny super-sensitive sensors record the changes in electrical activity in the brain from the magnetic field produced by the activity. MEGs are also noninvasive and have a high temporal resolution that is very similar to EEGs, making them better in this aspect compared to fMRI and PET scans.

Summary of Neuroimaging Findings

Temporal Structure and Experience

Music is naturally temporal in structure and experienced in a temporal dimension. Three temporal stages of processing music and language have been identified through MEG and EEG research. These stages seem to coincide well with different rates of walking and thinking. The three stages are outlined below:

1. First stage – up to 100 ms after audio starts, initial processing of physical properties of sound that make up music

2. Second stage – from 100- 200 ms, dealing with semi-automatic processing looking for unexpected cords or changes in repeated chords

3. Third stage – is further broken into three parts, 250 - 400 ms, 400 ms, and 600 ms. Processing for incongruities and music that is out of tune

Unexpected Bilaterialization

Neuroimaging, especially MEG analysis, surprisingly reveals both hemispheres of the brain are engaged covering a large area of the brain. These brain areas are engaged in a cooperative manner to process music. Initial expectations were that the right hemisphere would be the primary location for neural activity. While both hemispheres are involved in music processing, the left hemisphere is more involved when listening to music with a regular rhythm. Even more surprisingly, there is a great bilaterality in neural activity when listening to music than listening to speech. Research shows that language is strongly left-lateralized. It seems that instead of musical cognition being a special branch of language processing, the opposite is true. Language cognition developed from an innate musical cognition. Instead, music might originate from the more ancient roots of motor processing. When listening to music brain activity starts in the motor areas, aka the precentral gyrus at the top of the cerebral cortex, before spreading to other areas of the brain. The brain continues to maintain a heightened level of activity even after the music ends.

Innate Competency & Connection to Our Bodies

The innate musical cognition mentioned above is evidenced by studies of already music-competent newborns. One of the musical components that seem to be innately competent is rhythm. Researchers believe that distinct rhythms are interpreted even in newborns, because of the similarities to the rhythm of a beating heart, respiration, walking, and gestures. All animals move in a rhythmic or cyclical way, from the way humans walk to how jellyfish move about in the water. It seems the human experience is strongly linked with temporal understandings of our bodies and surroundings. Rhythm has such a powerful effect on humans, encouraging dance or movement. Again this is because of the strong link between musical competency and motor areas within the brain.

Musical Competency

Researchers found dissimilarities in brain matter density among different participants. It seems that beyond the innate competency for music, the brain can be trained to a higher level of music competency. One fMRI study of children found limited brain activity in the left hemisphere when listening to music. It’s likely that this lateralization develops later and matures with increased exposure to musical compositions. Certain elements of music, such as pitch have specific neural representations. Pitch is universally perceived across all cultures, suggesting that this element of music is pre-wired or predefined in the brain. Although pre-defined, exposure and music training can lead to an increased gray matter density. This very thing was observed among orchestral musicians, where Broca’s area in the left hemisphere had more gray matter, which is responsible for information processing. This increase in gray matter was not seen in participants that were not musically trained. It seems that areas of the brain that have an innate competence for music processing can become specialized and increase in neuronal tissue with proper training.

Combining Psychophysiological and Neurological Research

When looking at both of these papers, it’s clear that music elicits psychophysiological and neurological changes in humans. The degree of changes seems to largely depend on the music composition. There is a strong link between psychophysiological and neurological responses and rhythmic music. Current research suggests that the primary reason rhythmic music elicits neurological responses in the motor areas of the brain and psychophysiological responses in the form of body movements is the connection to our own bodily experiences. The human experience is temporal and a way to interpret this temporal existence is through rhythm. Our hearts beat rhythmically, we breathe rhythmically, and we walk rhythmically. The research reveals that music processing in the brain begins in the motor areas before spreading to both hemispheres. Beyond rhythm, musical expectation (and surprise) elicits strong neurological and psychophysiological responses. Our brains are primed to anticipate what is coming next, in music and in our daily experiences. This expectation can produce incredible tension as the music builds and swells to a climatic point. Deviation from the expected result induces a strong response that results in chills, increased heart rate, and increased skin conductance.

The strongest evidence for the emotivist position, where music can induce true emotions, are the similarities between psychophysiological and neurological responses measured when listening to music and those measured in real-life emotional situations. Arousing music with a positive affect elicits a change in heart rate, respiratory rate, skin conductance, muscle tension (facial expressions), and chills that is remarkably similar to the responses seen during happy experiences. It’s clear that musical emotions are connected to changes within the human body that can be measured using psychophysiological methods and neuroimaging. Music is a forceful regulator of emotions, capable of changing our bodies and activating our brains.