Abstract: The emotional information conveyed through phonological prosody and facial expressions forms the foundation for human interpretation of others' emotions and facilitates interpersonal interactions. Investigating how infants perceive, discriminate, and evaluate emotions embedded in these two modalities is conducive to deepening our understanding of infants' cognitive developmental characteristics and neural mechanisms. In this study, we conducted a comprehensive review of emotion in infants and found that the temporal and frontal cortex played an important role in the processing of emotional speech and facial expression in infants. Although emotional processing in infants involves both hemispheres of the brain, they have initially shown a right-hemisphere advantage similar to that of adults. Within the initial week after birth, infants demonstrate the capacity to distinguish emotional information and show a processing bias towards positive emotions. The positive bias toward emotion gradually transitions to the negative bias around 6 months of age. By 12 months of age, infants' negative emotional biases tend to stabilize, and they can understand the emotional meaning conveyed through speech and facial expressions, and guide their behavior accordingly. Based on these findings, we propose the "development theory of emotional bias." Importantly, visual-auditory multimodal information significantly contributes to infants' discernment and understanding of specific emotions.

Key words: infant, emotion processing, speech prosody, facial expression