Week 10: Physiology

This week we explore how the body responds to sound and music, from fast autonomic reactions to slower respiratory and motor changes. We will examine some physiological markers (heart rate and heart‑rate variability, skin conductance, respiration, and muscle tension), the physiological mechanisms that produce them (sympathetic/parasympathetic balance, entrainment), and methodological issues in measuring these signals in laboratory and concert settings.

Physiological Reactions¶

Physiology is the branch of biology that studies the functions and processes of living organisms and their parts (organs, tissues, cells). It explains how structures work, how they interact, and how systems maintain homeostasis. The term physiology originates from the Greek physis “nature, growth” and logos “study, account.” Thinkers as early as Aristotle described various human body functions but modern experimental physiology was first developed from the 17th century, with influential figures such as William Harvey (blood circulation) and Claude Bernard (the concept of the “milieu intérieur” or internal environment).

Autonomic mechanisms: sympathetic vs. parasympathetic¶

The autonomic nervous system (ANS) is at the core of human physiology and controls many automatic bodily functions. It has two main branches with generally opposite roles: the sympathetic branch mobilizes the body for action. This includes raising heart rate and blood pressure, increasing sweating, and redirecting blood to muscles. The parasympathetic branch functions via the vagus nerve and promotes rest and recovery. It is responsible for slowing the heart, aiding digestion, and supporting calm, restorative states.

Illustration from Anatomy & Physiology.

Physiologically, sympathetic effects are driven by noradrenergic signaling (norepinephrine) and tend to build over seconds to minutes. Vagal (parasympathetic) effects can act very quickly so they can be seen in millisecond‑level changes in heart rate. This difference in timing matters when you try to link musical events to physiological responses.

Performer physiology¶

Musicians’ physiology reflects both the immediate demands of performance and the social context in which it occurs. Heart rate and heart‑rate variability (HRV) shift with arousal, emotional valence and exertion: passages that are exciting, loud or physiologically demanding tend to raise heart rate and reduce short‑term HRV, whereas calm, slow passages often lower heart rate and increase vagal indices. In group settings—e.g. choirs and ensembles—cardiac patterns frequently show increased coherence or synchrony, an effect that scales with familiarity with the piece, empathic engagement, ensemble role and acoustic intensity.

Breathing is tightly coupled to musical structure and to vocal production, so respiratory measures are especially informative for singers and wind‑instrument players. Phrase boundaries, tempo and expressive demands shape inhalation timing, lung volume and the balance of rib‑cage and abdominal movement. These changes influence vocal physiology and electroglottographic signals. Performers also use breathing as a coordination signal—timed inhalations and exhalations cue entrances and phrasing and help align timing across an ensemble—so respiration recordings can reveal both individual performance strategies and group coordination dynamics.

Rhythm and meter provide a scaffold for sensorimotor coupling that links movement, breathing and cardiac timing. Musical pulses can entrain interbeat intervals (IBIs), gait‑like motions, and respiration such that physiological rhythms lock to beat or phrase structure; the strength of entrainment depends on rhythmic salience, motor engagement (e.g., tapping or conducting) and attentional focus.

Beyond autonomic timing, musical engagement evokes neurochemical and hormonal responses that shape affect and social bonding. Dopaminergic reward signaling and other neurotransmitter effects, oxytocin‑linked social affiliation and cortisol changes associated with stress or performance anxiety can all accompany strong musical experiences and modulate the magnitude and persistence of physiological synchrony.

It is important to note that contextual factors and individual differences critically determine physiological responses. Social variables (familiarity, empathy, hierarchy within an ensemble) and performer characteristics (training, fitness, attention and recent exertion) bias both direction and magnitude of responses.

Perceiver physiology¶

There is an increasing body of research that documents physiological responses to music among perceivers. Slow tempi, soft dynamics, smooth timbres and lower spectral centroid tend to promote parasympathetic dominance, including reduced heart rate, increased HRV, and lower tonic and phasic skin conductance. Fast tempi, greater loudness, sharp attacks and high‑energy spectral content favor sympathetic activation, including elevated heart rate, reduced HRV, and higher SCL.

As for performers, rhythmic regularity and tempo provide powerful entrainment cues for respiration and cardiac timing. Steady, slower musical pulses can slow breathing and enhance respiratory sinus arrhythmia (RSA), producing beat‑to‑beat vagal modulation of the heart, while faster rhythms typically accelerate respiration and heart rate and can reduce vagal indices; entrainment strength depends on rhythmic salience and the listener’s attentional engagement.

Emotional appraisal of music contributes independently via valence and arousal pathways. Music experienced as relaxing or pleasant biases toward parasympathetic engagement, whereas highly arousing, suspenseful, or threatening passages drive sympathetic responses.

As for performers, individual differences and situational factors determine response variability. Baseline autonomic tone (vagal tone), musical training, familiarity, cultural background, current fitness and affective state all alter sensitivity to musical features. So does social context, depent on how attention, expectation, memory and surprise change the magnitude and timing of autonomic responses. Furthermore, live or collective settings amplify shared autonomic dynamics and increase inter‑subject synchronization through social engagement and multimodal cues.

Interpersonal physiological responses¶

Due to advancements in measurement techniques, it has been possible to empirically study interpersonal physiological responses to music. As mentioned above, it is well-known that music can bring bodies into sync, both in performance and perception. A common beat can make people breathe together, and when breathing becomes shared, heart rate, skin conductance and even brain activity can start to look more similar across listeners or performers.

There are a few simple ways this happens. Sensorimotor entrainment (sensorimotor synchronization) means that moving, tapping or dancing together aligns bodily rhythms. Emotional contagion) describes how emotion spreads through a group and changes autonomic signals like heart rate and skin conductance. Social appraisal and hormonal responses—for example feeling connected or safe, sometimes linked to oxytocin—can also increase physiological similarity.

It is possible to measure such interpersonal responses by looking for time‑by‑time similarity between signals such as heart rate and respiration. Common tools are cross‑correlation (checking whether two signals rise and fall together, possibly with a delay), windowed coherence or windowed correlation (testing similarity in short time windows to see when coupling appears), and phase‑locking (checking whether rhythmic signals keep a stable phase relationship). Coupling tends to be stronger when people interact directly, pay attention to the same thing, or are emotionally engaged with the music.

Experiencing chills¶

In recent years interest has grown in musical chills (frisson), which are brief peak‑emotional responses often reported as goosebumps, shivers, or a cold sensation along the spine. These events are accompanied by transient autonomic signatures such as brief rises in skin conductance, short‑lived heart‑rate and respiration changes, pupil dilation and observable piloerection, and neuroimaging work links them to reward circuits (e.g., nucleus accumbens). Chills are typically triggered by salient musical features—unexpected harmonic shifts, climactic crescendos, intimate vocal timbres, or passages tied to personal memories—and they vary widely across listeners depending on personality, familiarity, context and attention.

Tingling sensations¶

Autonomous sensory meridian response) (ASMR) is another phenomenon that has received attention in recent years. It refers to a set of pleasurable, often tingling sensations and deep relaxation elicited by soft, intimate auditory and multimodal cues—whispering, close‑mic breathing, gentle tapping, slow movements and binaural spatialization—that overlap with some musical timbral and production techniques.

In sound and music research ASMR is relevant because its triggers highlight how micro‑acoustic features (low‑level dynamics, spectral detail, and spatial cues) and perceived interpersonal closeness modulate autonomic state. Many listeners report calming, parasympathetic effects (slower breathing, lowered heart rate) alongside transient markers such as piloerection and occasional phasic skin‑conductance rises.

When comparing ASMR to frisson/chills, note they can co‑occur but differ phenomenologically and physiologically: ASMR is typically soothing and prolonged, whereas frisson is brief and strongly reward‑linked, so analyses should treat them as distinct response classes and include timing, valence and arousal measures to disambiguate their profiles.

Measuring physiological reactions¶

There are numerous physiological measures that can be measured. Here we will look at some of the more popular ones that are used in music research.

Heart activity¶

There are two main types of measuring heart activity. The classic approach is to measure the electrical signal using electrocardiography ([ECG]). Electrodes placed on the chest pick up the electrical signal of each heartbeat, which can be described as a PQRST signal, where the distance between the R peaks (the R–R interval) describes the heart rate.

An illustration of a typical heart signal. From Wikipedia.

Nowadays, many people wear watches with built-in photoplethysmography (PPG), which measures the heart rate using optical sensors. These are flexible in that they can measure on the wrist, finger or ear. However, they are more sensitive to motion thatn ECG and less reliable for short‑term HRV or spectral analyses. In general, one should use ECG when needing a clean and high-quality signal, while PPG is sufficient for simple heart‑rate tracking.

Heart rate (HR) is conventionally reported in beats per minute (bpm). Typical resting adult HR is ~60–100 bpm (well‑trained athletes commonly 40–60 bpm; infants and children are considerably higher), and HR can change by a few to several tens of bpm with arousal, movement or exercise. When measuring heart rate, it is therefore important to measure a baseline heart rate and use within‑subject normalization to compensate for differences in resting HR and fitness level.

Heart‑rate variability (HRV) gives complementary information about autonomic balance. However, since it varies so much based on the heart rate, it is important to always to analyse it with respect to respiration and/or motion data to tell apart autonomic from metabolic or motor effects.

Respiration¶

Respiration is the record of how people breathe. The main respiration variables to look at are breathing rate (how many breaths per minute), tidal depth (how big each breath is), the timing of inhalation versus exhalation, and instantaneous breathing phase (where in the breath cycle you are at any moment). Typical resting adult values to report: breathing rate ≈ 12–20 breaths/min, tidal volume ≈ 400–600 mL per breath, and an inspiratory:expiratory ratio around 1:2 (inspiration ≈1.5–2.0 s, expiration ≈2.5–3.5 s at ~12 bpm).

A common respiration measurement device is the spirometer, which give precise measurements of air flow/volume. However, it requires a mouthpiece and are therefore not very practical for musical purposes. Then, a respiratory inductance plethysmography (RIP) belt is more practical as it can be fit around the chest. It captures respiration phase and timing well, but is less accurate for absolute long volume.

We have a large number of EQ02 LifeMonitor sensor vests from EquiVital that measure both respiration, heart rate, and accelerometry in one device.

Body temperature¶

Body temperature is not traditionally studied very much in music research. However, since our sensor vests can capture this information, it is relevant to explore it further. For medical applications, it is often common to measure body temperature rectally or tympanically. However, for musical purposes it is more relevant to measure skin temperature. Then, the relative temperature is more important than the absolute value. Also, since the skin temperature changes relatively slowly—over seconds to minutes—you should expect gradual trends rather than sharp, moment‑to‑moment peaks in response to musical events.

We have also began exploring capturing body temperature using thermal cameras. They have the benefit that you can capture a large group of people, such as a whole orchestra or audience group, using a single camera. Since these cameras only capture temperature, they also effectively anonymizes the data. Until recently, the resolution of such cameras was too poor to be useful. However, the latest cameras have full HD resolution, which makes it much more useful for various analysis.

Skin conductance¶

Skin conductance (also called electrodermal activity, EDA) is a simple way to index sympathetic arousal: when people are surprised, excited, or stressed, sweat‑gland activity changes and the skin conducts electricity differently. In recordings we separate a slow baseline level (skin conductance level, SCL) from quick event‑linked peaks (skin conductance responses, SCRs). In music studies you typically expect SCR peaks after surprising or high‑arousal moments, and SCL shifts when arousal remains elevated.

EDA is best captured from stable sites such as the palms, where the signal is strongest. The participant should sit still, because motion make the signal noisy. It is also important to record and report likely confounds—room temperature and humidity, skin hydration, electrode placement, movement, and relevant medications—and correct statistically for multiple trials or comparisons where needed.

Muscle tension¶

Electromyography (EMG) measures the tiny electrical signals produced by muscle activation and is commonly used to index the timing and amplitude of motor events. In music research it helps separate motor activity (playing an instrument, singing, tapping, or expressive facial movements) from autonomic responses like heart rate or skin conductance.

Typical EMG recordings use surface bipolar electrodes placed on the skin over the muscle of interest. Raw EMG is noisy, so preprocessing is important. It is also important to be aware how motion influences the result. However, with good filtering, it is possible to get meaningful data from EMG. At UiO, we have also explored how it is possible to use EMG data in various types of interactive music systems:

Comparison¶

Here is a comparison of the above mentioned signals, what they capture and their strengths and limitations.

At RITMO, most of our studies are based on comparing heart rate (from ECG), respiration (from RIP belts), temperature (from belts or thermal cameras), and muscle activation (from EMG). We typically also always record these in combination with accelerometry and audio/video documentation so that we are able to correct for contextual factors.

None of the physiological signals are easy to work with, so it requires a lot of pre-processing and analysis to get meaningful results. The benefit is that we can learn much more about human experience of sound and music, with signals that are at least easier to interpret than brain measurements.

Questions¶

1. What are the differences between heart rate and heart variability?

2. How would you design an experiment to test cardiac and respiratory entrainment to tempo changes in live performance?

3. Which objective (physiological) and subjective measures would you combine to distinguish frisson (chills) from ASMR?

4. What environmental and participant confounds must be controlled when using thermal imaging or skin temperature to infer autonomic responses?

5. How would you analyze respiration to avoid misinterpreting the data?