when do babies become able to distinguish different speech sounds
Phonological development Infants as young as 1 month perceive some speech sounds as speech categories (they display categorical perception of speech). For example, the sounds /b/ and /p/ differ in the amount of breathiness that follows the opening of the lips. Using a computer generated continuum in breathiness between /b/ and /p/, Eimas et al. (1971) showed that English-learning infants paid more attention to differences near the boundary between /b/ and /p/ than to equal-sized differences within the /b/-category or within the /p/-category.[4] Their measure, monitoring infant sucking-rate, became a major experimental method for studying infant speech perception.
Obligate nasal breathing Human infants are commonly described as obligate nasal breathers as they prefer breathing through their nose rather than mouth.[3] Most infants, however, are able to breathe through their mouth if their nose is blocked.[3] There are however certain infants with conditions such as choanal atresia in which deaths have resulted from nasal obstruction.[3] In these cases there are cyclical periods of cyanosis. The infant initially attempts to breathe through the nose, and is unable to; hypercapnia occurs, and many babies instinctively begin to cry. While crying, oral ventilation occurs and cyanosis subsides. There is variation in the length of time until a baby begins oral breathing, and some will never cease attempts at nasal breathing. It has also been suggested that infants may not be able to sustain oral breathing for significant lengths of time, because of the weakness of the muscles required to seal the nasal airway and open the oral airway.[3]