Affective responses to the acoustic features of sounds from a Polyvagal Perspective (excerpt, conclusion only)
Body sounds have unique acoustic features and elicit unique subjective ratings. Acoustically, body sounds were the least separable and had the highest temporal modulation. These findings become more intuitive and understandable, once we review examples of sounds that are high and low on separability and temporal modulation.
Music, as a category of acoustic, is highly separable. When we listen to music our nervous system anticipates the rhythmic changes and functionally has the capacity to fill in short gaps. This feature of separability is the psychoacoustic basis for music compression algorithms (e.g. MP3 players). In contrast, body sounds are not acoustically separable. This feature of low separability in body sounds is processed by our nervous system as less predictable and may place us into a behavioral state of hypervigilance. Functionally, this result in many individuals having great difficulty in ignoring the body sounds of others. Since music, in general, is prosodic and separable, both the modulation of frequencies that define prosody and the predictability of the sound sequences in combination signal our nervous system to calm and not to be vigilant. Of course, when music is less predictable and less prosodic, it loses its ability to calm. However, even when music is up-tempo (e.g., dances, marches) it maintains sufficient separability and prosody to maintain prosocial behaviors while arousing and mobilizing. In contrast, the acoustic features of body sounds by lacking prosody and predictability trigger a state of hypervigilance, which would disrupt ongoing tasks and social interactions.
The temporal modulation describes the relative acoustic energy associated with rhythmic changes in intensity (i.e., temporal) compared to frequency modulation (e.g., prosody). The sounds of footsteps is an example of an IADS sound with high temporal modulation. With footsteps there are short bursts of acoustic energy that rhythmically occur within a relatively narrow frequency band. Body sounds, similar to footsteps, frequently have a rhythmic component (e.g., coughing, flatulence, regurgitation, lip smacking, chewing) generating sounds within a narrow frequency band. The acoustic energy in body sounds is biased towards temporal modulation relative to frequency modulation, the acoustic component linked to prosody and associated with sounds that are calming and signal safety.
When the acoustic qualities that constitute body sounds are perceived, our nervous system promotes hard-wired subjective responses. The IADS ratings provide a window into these hard-wired subjective responses of pleasure and arousal to body sounds. Subjectively, body sounds uniquely were experienced as low on both the pleasure and arousal dimensions. This contrasts to music, which was experienced as high on both dimensions. The natural sounds shared with music a high degree of arousal and shared with body sounds a low rating of pleasure. Focusing first on the arousal dimension, body sounds uniquely had noticeably lower ratings on arousal. Arousal may be interpreted as a shift in physiological state towards an increase in activation of the sympathetic nervous system that would promote movement. Functionally, increased arousal would be associated with movements both towards positive and negative stimuli. These data suggest body sounds are not only perceived as negative (i.e., low on the pleasure scale), but they are associated with reduced mobilization or even perhaps immobilization. In contrast both music and natural sounds functionally stimulated the neural circuits that would support movement, although the pleasurable rating would suggest that the direction of the movement may be different for music and natural sounds. The high pleasure rating of music would suggest movement towards others (e.g., prosocial) observed in dancing and marching, while the low pleasure rating of natural sounds would suggest more of a defensive fight-flight response. Although the average subjective response to natural sounds is low on the pleasure scale, closer inspection identifies a great range of responding. There is a subset of natural sounds that are rated as pleasurable and arousing. These sounds, more similar to music, may function similar to music in their ability to support prosocial movement.
Across all IADS sounds greater temporal modulation was associated with lower subjective ratings of arousal. Extrapolating to body sounds, which had high temporal modulation and low arousal, suggests that body sounds may be triggering a biobehavioral “shut down” reaction normally associated with of life threat. The data demonstrate that body sounds have a unique acoustic profile that may trigger hard-wired responses, which may be especially pronounced in individuals who are in a physiological state of hypervigilance.
Future research will focus on three areas: 1) evaluate acoustic properties (i.e., separability, temporal modulation) of a broad range of sounds that are irritating; 2) evaluate individuals with sound sensitivities to determine the role that an individual’s physiological state and middle ear transfer function has in determining subjective, behavioral, and physiological reactions to irritating sounds; 3) Reducing auditory hypersensitivities and normalizing the middle ear transfer function with the Listening Project Protocol (see Porges et al., 2013, 2014)
1The middle ear transfer function objectively quantifies the acoustic ‘permeability’ to sounds in different frequency bands (Porges & Lewis. 2011). An atypical middle ear transfer function could functionally amplify the acoustic features of body sounds and attenuate frequency modulations in pleasing sounds. If this is the case, then the middle ear transfer function could be rehabilitated through the Listening Project Protocol.
Dr. Porges is currently a Distinguished University Scientist, at Indiana University Bloomington (and is formerly a professor in the Department of Psychiatry at the University of North Carolina in Chapel Hill, North Carolina). Prior to moving to North Carolina, Dr. Porges directed the Brain-Body Center in the Department of Psychiatry at the University of Illinois at Chicago, where he also held appointments in the Departments of Psychology, BioEngineering, and the Program in Neurosocience.