The Vocal-Vagal Hypothesis

The Voice as a Vagal Communication Channel

Humans and other mammals spontaneously produce and respond to the emotional tone in vocal communications -- independent of linguistic content. This capacity would presumably have adaptive function for both affiliative and defensive social behaviors. We posit that vagal innervation of the vocal apparatus is a key component of this communication mechanism. Premotor parasympathetic neurons in Nucleus Ambiguus (NAmb) send projections through the ventral branch of the vagus to modulate the tone of muscles involved in vocal production. We hypothesize that this alters acoustic signals of the voice, such as the average pitch, pitch modulation statistics, acoustic resonance, and vibrato. These acoustic signals could encode information about the global autonomic state of a speaker and thus, indirectly, their emotional state and social intentions. Vagal sensory processes in the auditory periphery could potentially underlie autonomic mirroring, by detecting acoustic cues correlated with the vagal tone of the vocalizer and directly modulating vagal tone of the listener.
We are testing this by recording autonomic physiology and vocal signals during spontaneous speech production and passive speech reception.

Spectral Features of Vocal Signals.
Left: Features in the spectrogram of vocalizations, such as harmonic stacks, duration of phonations, small oscillations in pitch (vibrato), and distribution of pitches over time, may encode information about vagal tone of the vocalizer.
Right: These features can be extracted using the Modulation Power Spectrum.

Mechanism of Therapeutic Effects of Singing

We further hypothesize that long slow pressurized exhalation coupled with prosodic vocalization will increase vagal activity through proprioceptive parasympathetic circuits. If so, singing would be a low-tech, non-invasive method for increasing vagal tone, with potential therapeutic applications as well as interesting cultural implications.
We are testing this by recording autonomic physiology in singers.

Using Respiratory Sinus Arrhythmia to Measure Vagal Tone During Singing A. Breathing at rest, as measured by chest expansion. B. Electrocardiogram (ECG) recorded simultaneously with panel A. C. Sound waveform during vocalizing. D. Breathing measured simultaneously with C (vertical scale same as in A). Notice the inhalations are faster (steeper slope) and greater amplitude than at rest, while exhalations (corresponding to the vocal phrases in C) are much longer in duration. E. ECG recorded simultaneously with panel C and D. F. Using data like those shown in A-E we computed the heart rate relative to time in the breathing cycle, averaged over many breath cycles. Here subject was recorded before, during and after 60 minutes of singing. At rest, the heart rate had no trend over the breath cycle (black curve is flat), indicating low vagal tone. During vocalization the heart rate slowed down during each exhalation (red, green and blue curves slope downward), indicating increased vagal activity. On a slower timescale, the average heart rate gradually declined over 60 minutes of singing (compare red curve to blue curve). At rest after singing, the overall heart rate was much lower than before (magenta curve is below black curve) and Respiratory Sinus Arrhythmia persisted (magenta curve slopes downward).