Language is a unique human trait and a vital means for communication. However, it is unclear how the newborn's brain perceives language and speech sound. It is of great significance to confirm the typical brain mechanism of speech sound perception in neonates. Language with sound carrier is called speech sound, which is produced by human vocal organs. Since newborns are not able to recognize words, neonatal processing of language is generally referred to the processing of speech. With the development of neuroscience, non-invasive neuromeasurement techniques have been increasingly applied to study the brain mechanisms of speech processing in infants and even newborns. This article introduced the brain mechanisms of mother tongue and foreign languages speech perception in newborns from four aspects, i.e., the processing of phonetic structures, phonetic physical attributes, phonetic units and phonetic categories. For the processing of phonetic structures, a number of studies have found that humans are born with the fundamental mechanisms for encoding the order and structure of syllables in speech sequences. It has been shown that newborns have better encodings for the syllables at the edges of sequences than those in the middle. They are able to detect the repetition structures and their positions in syllable sequences. Besides, the statistical learning ability for linguistic sequences is already present at birth. With regard to the processing of physical attributes of speech, significant MMR (mismatch response) are found in response to the change of phonetic physical attributes at the frontal lobe and temporal lobe of newborns. The MMR is the infant equivalent of the mismatch negativity (MMN) in the adult brain. For the processing of phonetic units, the neonatal brain can also elicit the MMR in response to the change of phonetic units. With respect to the processing of phonetic categories, humans are born with a left hemisphere superiority in processing their mother tongue, whereas the right hemisphere is more sensitive to non-native languages. In conclusion, human beings have relatively complete speech processing mechanisms in the neonatal period, which mainly involves temporal lobe and the Broca's area of inferior frontal gyrus. The left inferior frontal gyrus plays an important role in detecting speech structure, while the right superior temporal gyrus is responsible for distinguishing the prosody of the speech.

According to the existing research reviewed above, three questions needing further research and explorations are proposed. Firstly, the polarity of MMR evoked by novel auditory materials needs to be further studied. Longitudinal experiments are suggested to be conducted in the future to reveal the key developmental stage of polarity change of MMR in infants. Secondly, we also suggest that meta-analysis of speech processing effects in different brain regions should be carried out to further reveal the brain mechanism of speech perception in neonates and infants. Lastly, most of the existing studies on the brain mechanism of neonatal speech processing are single time node studies, and do not investigate the obvious changes of the brain stimulated by speech stimulation in the environment of neonatal dependence after birth. In the follow-up study, high spatial resolution technique (fMRI/fNIRS) can be used to further explore the key brain regions of neonatal speech learning.