Perceptual Development: Speech Perception

Development of Phonological Perception

From birth, infants appear to hear speech differences in a categorical manner. They hear many, if not all, of the sounds of the world's languages and appear to notice certain critical differences from the earliest ages tested. For example, the acoustic difference between the /b/ in big and the /p/ in pig is subtle. The principle change has to do with the relationship between when the sound begins and when the vocal cords vibrate. In /b/ the vocal cords and sound begin simultaneously, whereas in /p/ the vocal cords begin their vibration noticeably later. This time difference is called the voice onset time (VOT). In the 1960s, Peter Eimas and colleagues systematically varied VOT to demonstrate that infants pay more attention to the difference between /b/ and /p/ rather than a similar-sized VOT difference between one /p/ and another /p/ (this time said with even more time between the onset of the sound and when the vocal folds vibrate).

How did they show this? They took advantage of something all infants do: suck. Using a method called the sucking procedure, these researchers gave infants an artificial nipple attached to a device that registered each time the infant sucked. In this case, the researchers played a “pa” over and over for English-learning infants. As the infants succumbed to boredom, their sucking decreased. When the audio was switched to “ba,” their sucking increased, suggesting infants noticed the difference between the two sounds. Infant's sucking did not recover if the /p/ with a larger VOT was played, even though the size of the acoustic difference was the same. Patricia Kuhl subsequently demonstrated that even chinchillas make this distinction, suggesting that categorical perception of speech is not specific to humans and may relate to hardwired abilities of the nervous system to hear certain acoustic changes over others.

Infants even notice sound distinctions not found in their native language. For example, Japanese speaking adults have a hard time hearing the difference between /r/ and /l/, which leads them to confuse “surprise” with “supplies.” Yet Japanese infants can hear the difference, and a wealth of research indicates this is true for a wide range of difficult-to-hear contrasts across a number of languages. This ability to distinguish difficult, non-native contrasts disappears by approximately 12 months. Such results suggest that though children can still learn new language sounds, around one year of age they have begun to specialize in their native language and have begun to ignore distinctions that are not important for that language. Kuhl called this focusing on the important sounds the perceptual magnet effect. That is, the native sound acts like a magnet: pulling similar non-native sounds to it (making them being heard the same).

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