Humans need auditory experience to produce typical volitional nonverbal vocalizations

Nonverbal vocalizations, such as screams and cries, are a fundamental part of human communication, believed to predate language in our evolutionary history. While some, like infant cries, appear innate, many emotional vocalizations in adults are volitional, suggesting a learned component. Unlike reflexive vocalizations, these volitional expressions might require auditory input for typical development, similar to speech acquisition. This study hypothesized that auditory deprivation would lead to atypicalities in volitional vocalizations, particularly in expressing emotions like aggression and pain, but not necessarily fear. This hypothesis was tested by comparing the acoustic properties and perceptual judgments of volitional vocalizations produced by profoundly deaf adults and hearing controls.

Existing research suggests that human nonverbal vocalizations share similarities across cultures and with animal calls, suggesting a possible innate basis. However, the role of auditory experience in shaping these vocalizations remains debated. While infant cries appear innate, the acoustic structure of cries may become atypical with age in deaf individuals, suggesting ongoing auditory feedback is necessary. Studies on laughter have yielded mixed results, with some suggesting a relatively typical structure in deaf adults, while others report variations. Previous studies on hearing-impaired adults have shown some structural abnormalities in volitional vocalizations, but the impact of complete sensory deprivation was not fully understood. The current study aimed to address this gap by examining a large sample of profoundly deaf adults with varying degrees of auditory experience.

This study employed a mixed-methods approach, combining acoustic analysis and perception experiments. One hundred and twenty adults participated: sixty profoundly deaf adults (including twenty with zero auditory experience) and sixty age- and sex-matched hearing controls. Participants produced volitional vocalizations expressing aggression, pain, and fear, which were recorded and analyzed acoustically. Fifteen acoustic parameters were measured, including fundamental frequency, formant frequencies, amplitude, duration, perturbation, and nonlinear phenomena. Four independent perception experiments with 444 listeners assessed emotion identification accuracy, perceived authenticity, and the detectability of deafness from the vocalizations. Linear mixed models (LMMs) and generalized linear mixed models (GLMMs) were used to analyze the data, controlling for various factors such as sex and listener bias. Principal component analysis (PCA) and discriminant function analysis (DFA) were employed to reduce dimensionality and examine acoustic distinctiveness.

Acoustic analyses revealed significant differences between deaf and hearing adults in their vocalizations. Deaf adults produced aggressive and pain vocalizations with unusually high pitch, fewer nonlinear phenomena (resulting in a more tonal quality), and wider formant spacing (indicating shorter apparent vocal tract length). These atypical acoustic features were particularly pronounced in aggression and pain vocalizations. In contrast, fear vocalizations were comparatively typical. Perception experiments confirmed these findings. Listeners were less accurate in identifying the intended emotions of deaf vocalizers, particularly aggression, and rated their vocalizations as less authentic. Importantly, listeners could readily detect deafness from the vocalizations alone. The degree of auditory atypicality correlated with the extent of auditory deprivation, with congenitally deaf adults with zero auditory experience exhibiting the most atypical vocalizations.

The results strongly support the hypothesis that auditory experience is crucial for the typical development of volitional nonverbal vocalizations, particularly for expressing aggression and pain. The lack of typical acoustic features in deaf vocalizers' calls (e.g., low pitch, harshness, narrow formant spacing for aggression) likely explains the listeners' difficulty in correctly interpreting the intended emotions. The findings suggest that the capacity to voluntarily control the expression of emotions through vocalizations involves vocal learning, requiring auditory input throughout development, much like the acquisition of speech. The relatively typical acoustic structure and perception of fear vocalizations might be due to a more innate, less learned basis or a coincidental overlap with the acoustic profile of a fear call in the deaf population. The study highlights the interplay between auditory input, vocal motor control, and the development of expressive communication.

This study provides compelling evidence that auditory experience is necessary for the typical development of volitional nonverbal vocalizations in humans, especially those expressing aggression and pain. This suggests that vocal learning plays a significant role in acquiring the full range of human expressive vocalizations. Future research should investigate the relative contributions of auditory input and feedback, vocal production experience, and potential social factors in shaping these vocalizations. Understanding these processes is crucial for developing effective communication strategies for individuals with hearing impairments.

This study focused on negatively valenced emotions; further research is needed to examine positive vocalizations. The use of posed vocalizations might not perfectly reflect spontaneous expressions; future studies could explore vocalizations in natural settings. While the study controlled for several factors, other potential influences such as the frequency of vocalization and social stigma remain to be explored. Further research with deaf-blind individuals would help determine the role of other sensory modalities.