![]() Using the sucking technique, this study revealed that at birth, infants' discrimination of /pa/ and /ba/ was categorical not only with the perception of sounds in their native language but also with sounds from foreign languages as if the infants heard all the phonetic distinctions used in all languages. If so, young infants could not be expected to show it, while older infants, who had experienced language, might be expected to do so. This prompted researchers to question if categorical perception was the result of experience with language. The discovery that categorical perception was language-specific suggested that it might be a learned behavior. However, adult listeners could do this only for sounds in their native language. As predicted by the categorical perception phenomenon, their discrimination improved at the boundary between the two phonetic categories. To measure categorical perception, adults were asked to discriminate between a series of sounds varying in equal steps in acoustic dimension from /ra/ to /la/. ![]() However, to understand speech, more than the ability to discriminate between sounds is needed speech must be perceptually organized into phonetic categories, ignoring some differences and listening to others. When the head is turned during the presentation of the comparison stimulus, the child is rewarded with a visual stimulus of a toy which makes a sound.Īs a result of studies using these techniques, it has been shown that infants at the earliest ages have the ability to discriminate phonetic contrasts (/bat/ and /pat/) and prosodic changes such as intonation contours in speech. With this technique, a child is trained to turn his or her head when a speech sound, repeated once every second as a background stimulus, is changed to a comparison speech sound. ![]() Head turn conditioning is used to test infants between 6 months and one year of age. In HAS, infants from 1 to 4 months of age suck on a pacifier connected with a pressure transducer which measures the pressure changes caused by sucking responses when a speech sound is presented. ![]() To understand how bottom-up processing works in the absence of a knowledge base providing top-down information, researchers have studied infant speech perception using two techniques: high-amplitude sucking (HAS) and head-turn (HT). Perception occurs when both sources of information interact to make only one alternative plausible to the listener who then perceives a specific message. In the latter, we use stored information about language and the world to make sense of the speech. In the former, we receive auditory information, convert it into a neural signal and process the phonetic feature information. In addition to the acoustic analysis of the incoming messages of spoken language, two other sources of information are used to understand speech: "bottom-up" and "top-down". Two other distinct aspects of perception -segmentation (the ability to break the spoken language signal into the parts that make up words) and normalization (the ability to perceive words spoken by different speakers, at different rates, and in different phonetic contexts as the same) -are also essential components of speech perception demonstrated at an early age by infants. Speech prosody (the pitch, rhythm, tempo, stress, and intonation of speech) also plays a critical role in infants' ability to perceive language. It has been demonstrated how certain physiologic gestures used during speech produce specific sounds and which speech features are sufficient for the listener to determine the phonetic identity of these sound units. Since the 1950s, great strides have been made in research on the acoustics of speech (i.e., how sound is produced by the human vocal tract). Other research has shown the strong effect of environment on language acquisition by proving that the language an infant listens to during the first year of life enables the child to begin producing a distinct set of sounds (babbling) specific to the language spoken by its parents. Studies of infants from birth have shown that they respond to speech signals in a special way, suggesting a strong innate component to language. Speech perception, the process by which we employ cognitive, motor, and sensory processes to hear and understand speech, is a product of innate preparation ("nature") and sensitivity to experience ("nurture") as demonstrated in infants' abilities to perceive speech. The ability to hear and understand speech.
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