Abstract-Synchrony refers to individuals' temporal coordination during social interactions. The analysis of this phenomenon is complex, requiring the perception and integration of multimodal communicative signals. The evaluation of synchrony has received multidisciplinary attention because of its role in early development, language learning and social connection. Originally studied by developmental psychologists, synchrony has now captured the interest of researchers in such fields as social signal processing, robotics and machine learning. This paper emphasizes the current questions asked by synchrony evaluation and the state-of-the-art related methods. First, we present definitions and functions of synchrony in youth and adulthood. Next, we review the non-computational and computational approaches of annotating, evaluating and modeling interactional synchrony. Finally, the current limitations and future research directions in the fields of developmental robotics, social robotics and clinical studies are discussed.

Various aspects of motherese also known as infant-directed speech (IDS) have been studied for many years. As it is a widespread phenomenon, it is suspected to play some important roles in infant development. Therefore, our purpose was to provide an update of the evidence accumulated by reviewing all of the empirical or experimental studies that have been published since 1966 on IDS driving factors and impacts. Two databases were screened and 144 relevant studies were retained. General linguistic and prosodic characteristics of IDS were found in a variety of languages, and IDS was not restricted to mothers. IDS varied with factors associated with the caregiver (e.g., cultural, psychological and physiological) and the infant (e.g., reactivity and interactive feedback). IDS promoted infants’ affect, attention and language learning. Cognitive aspects of IDS have been widely studied whereas affective ones still need to be developed. However, during interactions, the following two observations were notable: (1) IDS prosody reflects emotional charges and meets infants’ preferences, and (2) mother-infant contingency and synchrony are crucial for IDS production and prolongation. Thus, IDS is part of an interactive loop that may play an important role in infants’ cognitive and social development.