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. their measure, monitoring infant sucking-rate, became a major experimental method for studying infant speech perception.