Faces and Voices Processing in Human and Primate Brains: Rhythmic and Multimodal Mechanisms Underlying the Evolution and Development of Speech

Abstract

While influential works since the 1970s have widely assumed that imitation is an innate skill in both human and non-human primate neonates, recent empirical studies and meta-analyses have challenged this view, indicating other forms of reward-based learning as relevant factors in the development of social behavior. The visual input translation into matching motor output that underlies imitation abilities instead seems to develop along with social interactions and sensorimotor experience during infancy and childhood. Recently, a new visual stream has been identified in both human and non-human primate brains, updating the dual visual stream model. This third pathway is thought to be specialized for dynamics aspects of social perceptions such as eye-gaze, facial expression and crucially for audio-visual integration of speech. Here, we review empirical studies addressing an understudied but crucial aspect of speech and communication, namely the processing of visual orofacial cues (i.e., the perception of a speaker’s lips and tongue movements) and its integration with vocal auditory cues. Along this review, we offer new insights from our understanding of speech as the product of evolution and development of a rhythmic and multimodal organization of sensorimotor brain networks, supporting volitional motor control of the upper vocal tract and audio-visual voices-faces integration.

Multimodal imitative learning and synchrony in cetaceans: A model for speech and singing evolution

“…In the case of humans, evidence has shown that rather than being an exclusively innate ability, imitation capacity develops during infancy and childhood, supported by the maturation of sensorimotor brain networks and domain-general associative learning of multimodal information, both fostered by socially rewarding interactions. This network represents a suitable candidate to coordinate the processing of visual information and the execution of the corresponding motor sequence required for the imitation of facial expressions, such as lip or tongue protrusion (see Michon et al, 2022 ). Accordingly, recent empirical studies and meta-analyses have challenged the widely assumed view that imitation is an innate skill that emerges independent of environmental contingencies, in both human and non-human primate neonates, indicating other forms of reward-based learning as relevant factors in the development of social behavior (see Heyes, 2021 ).…”

“…Accordingly, recent empirical studies and meta-analyses have challenged the widely assumed view that imitation is an innate skill that emerges independent of environmental contingencies, in both human and non-human primate neonates, indicating other forms of reward-based learning as relevant factors in the development of social behavior (see Heyes, 2021 ). Recently, Michon et al, 2022 reviewed empirical studies offering new insights from their understanding of speech as the product of evolution and development of a rhythmic and multimodal organization of sensorimotor information, supporting volitional motor control of the upper vocal tract and audio-visual voices-faces integration, and proposed that human imitation relies on crossmodal associations of sensorimotor information (e.g., visuomotor associations for facial imitation and audiomotor associations for vocal imitation) that develop along with social interactions and sensorimotor experience during infancy and childhood.…”

“…Alternative and even narrower definitions have been proposed for imitation, for example arguing that the only form of true imitation that is found on higher-level cognitive skills, called “production learning,” is when the action of the model is entirely “novel” to the observer ( Zentall, 2003 , 2006 ; Hoppitt and Laland, 2013 ; Heyes, 2021 ). However, it is important to note that when defining novelty, there will always be some degree of similarity to what the observer has done before ( Whiten et al, 1996 ) and that the imitation of novel actions could still be possible by the same learning mechanisms as familiar actions (see Heyes, 2021 and Michon et al, 2022 for review).…”

“…Moreover, this view of vocal imitation as “less complex” is somewhat surprising since this capacity is a hallmark of human spoken language, which, along with motor imitation and other advanced cognitive skills, has certainly fuelled the evolution of human culture. In addition, cross-modal interaction between gestures (especially facial gestures) and vocal communication is highly relevant in speech development and most likely it was so in the evolutionary origins of speech ( Michon et al, 2022 ).…”

“…In cetaceans, this has been supported by Murayama et al (2017) who reported that a beluga whale was capable of spontaneous establishment of cross-modal stimulus equivalence between visual and auditory symbols (novel objects that were untrained). It is important to note that until this study the capacity for cross-modal stimulus equivalence was considered exclusively human and attributed to the linguistic faculty ( Michon et al, 2022 ). Similarly, the results on deferred imitation in orcas mentioned above, which also include distractions during the retention interval before the copy command was given, gives further support to this interpretation ( Zamorano-Abramson et al, 2023 ).…”