Conventional wisdom says that human perceptual and cognitive functions broaden and improve as humans grow and mature, but a study published in the June 17 issue of the Proceedings of the National Academy of Sciences provides evidence that the opposite occurs in infant perception of audiovisual speech.
Researchers from the Charles E. Schmidt College of Science at Florida Atlantic University and the Universities of Barcelona and Pompeu Fabra in Spain studied English-learning infants in the United States and Spanish-learning infants in Spain to determine if they can perceive people’s facial speech gestures and accompanying vocalizations as part of the same event.
The ability do this is crucial for adaptive communication. Dr. David J. Lewkowicz, professor of psychology in FAU’s College of Science and head of the Infant Development Laboratory, and his colleagues, Drs. Ferran Pons, Salvador Soto-Faraco and Núria Sebastián-Gallés from Spain, hypothesized that younger infants may actually be better at integrating facial speech gestures and vocalizations than older infants and that the developmental decline in this ability may be due to increasing specialization for native-language phonology as infants learn their own speech and language.
“Our hypothesis is contrary to conventional wisdom because it assumes that perceptual abilities improve as infants develop,” said Lewkowicz. “Consistent with our hypothesis, we found that younger infants could integrate the facial and vocal gestures of foreign speech sounds, but that older infants no longer did.”
The results of this study provide the first evidence that perception of audiovisual non-native speech narrows during infancy precisely during the time that infants are acquiring their native language phonological system. These researchers demonstrate that the perceptual system becomes gradually tuned to key native-language audio-visual correspondence, and as it does so, sensitivity to the phonetic information inherent in foreign language sounds declines.
To investigate their hypothesis, the researchers presented infants with speech syllables (/ba/ and /va/) that are distinguishable to English speakers but not to Spanish speakers. During the experiment, infants first watched side-by-side videos of the same person silently and repeatedly uttering a /ba/ syllable on one computer monitor and a /va/ syllable on the other monitor. Results of this test showed that the infants did not prefer one syllable over the other. Then, the researchers allowed the infants to listen to either the /ba/ or the /va/ syllable for 45 seconds and then tested their preferences for the visual syllables again by showing the silent versions of the side-by-side syllables. This time, the six-month-old Spanish infants showed a clear preference for the visible syllable that matched the audible syllable that they had just heard, indicating that they perceived them as belonging together. In contrast, the 11-month-old Spanish infants did not show such a preference indicating that they did not perceive the audible and visible syllables as belonging together. This latter finding was consistent with the fact that the /v/ sound does not exist in Spanish and with the idea that the older Spanish-learning infants’ greater experience with the Spanish language reduced their sensitivity to the visible and audible sounds of other languages.
Unlike the decline in the ability to integrate auditory and visual speech in Spanish-learning infants, the researchers found that both six-month-old and 11-month-old English-learning infants successfully matched the audible and visible syllables. In addition, the researchers confirmed that the decline in the perception of non-native audiovisual speech persists into adulthood by showing that Spanish adults were unable to integrate the same syllables that were presented to the infants, but that English adults easily did.
“It is important to emphasize that the perceptual narrowing that we found does not reflect a complete loss of perceptual sensitivity to non-native sensory inputs,” said Lewkowicz. “Rather, it reflects a reorganization of perceptual mechanisms that then leads to decreased sensitivity to non-native sensory inputs.”
Florida Atlantic University opened its doors in 1964 as the fifth public university in Florida. Today, the University serves more than 26,000 undergraduate and graduate students on seven campuses strategically located along 150 miles of Florida's southeastern coastline. Building on its rich tradition as a teaching university, with a world-class faculty, FAU hosts ten colleges: College of Architecture, Urban & Public Affairs, Dorothy F. Schmidt College of Arts & Letters, the Charles E. Schmidt College of Biomedical Science, the Barry Kaye College of Business, the College of Education, the College of Engineering & Computer Science, the Harriet L. Wilkes Honors College, the Graduate College, the Christine E. Lynn College of Nursing and the Charles E. Schmidt College of Science.
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