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 deepens our understanding of their cognitive development and neural mechanisms. Infants’ emotional processing primarily relies on facial expressions, speech, and cross-modal sensory processing involving both visual and auditory inputs.
In emotional facial processing, the temporal and frontal cortex are the core brain regions. Although emotional processing in infants involves both brain hemispheres, the right hemisphere appears to have an advantage. Remarkably, just 36 hours after birth, newborns can distinguish and imitate facial expressions, demonstrating their sensitivity and interactive ability with the surrounding environment. Two-day-old newborns can generally distinguish different facial expressions, such as happiness, sadness, surprise, and fear. By 3~4 months of age, infants can reliably differentiate between various facial expressions. At 5 months, they begin to distinguish different types and forms of facial expressions, including dynamic ones. After 6 months, infants exhibit classification perception of emotional facial expressions. By 7 months, they are very sensitive to dynamic facial expressions. By 8 months and beyond, infants start to show an understanding of emotional facial expressions, with positive emotions being understood earlier than negative ones. Although the global brain network for infant emotional facial perception is not yet fully mature at this stage, local specific brain networks have developed to a level almost equivalent to those of adults.
In infants’ emotional speech processing, the temporal cortex is the core brain area. During the first week after birth, babies exhibit considerable sensitivity to emotional speech and can generally distinguish different emotional tones, which helps improve their interactions with others. By two months of age, infants can distinguish between happy and neutral speech, showing a greater sensitivity to happy voices. At 5 months, infants are able to differentiate between happy, angry, and neutral voices, again displaying a preference for happy voices. By 7 months, infants can distinguish between happy, sad, angry, and neutral voices, with sensitivity to angry and sad tones.
Babies exhibit the ability to process emotional information across multiple modalities, demonstrated by their capacity to match and transfer emotional information across different sensory modalities. These abilities gradually improve with age. Compared to the processing of emotional speech and facial expressions, the development of cross-modal emotional processing in infants occurs later.
Emotional bias manifests differently at various stages of infant development. Infants predominantly exhibit positive emotional processing in the first 6 months, but this gradually shifts to a stable negative bias after 6 months. Based on this observation, we propose the “Developmental Theory of Emotional Bias”: Human emotional processing biases change around six months of age, with positive biases observed in infants aged 6 months and below, and increasingly stable negative biases observed in infants aged 6 to 7 months and beyond. From a cognitive development perspective, the infants’ brain prioritizes processing stimuli relevant to their developmental stage. For infants aged 6 months and below, a more positive response to positive emotions may help establish strong connections with parents and ensure more care, thus favoring positive emotions. After 6 months, as infants develop motor abilities such as crawling, running, and jumping, they begin to actively explore the world. At this stage, they need to be more sensitive to threatening information to protect themselves from harm. Therefore, infants at this stage prefer negative emotions.
Overall, as infants age, their ability to process emotions gradually improves, encompassing multiple levels from identifying and distinguishing emotions to capturing emotional changes and eventually understanding and applying emotions. Currently, research on visual, auditory, and cross-modal emotional information processing during infancy has made significant progress and established a relatively solid foundation. Future research needs to systematically investigate emotional processing from infancy through early childhood to construct a complete developmental timeline and reveal in-depth changes in infant emotional development. Additionally, specific experimental designs must rigorously control for additional variables and combine multiple research methods to provide more convincing evidence for understanding infant cognitive development and brain mechanisms.
