An international team of researchers have found evidence challenging conventional wisdom that human perceptual and cognitive functions broaden and improve as they grow and mature.
The team has studied English-learning infants in the US and Spanish-learning infants in Spain.
The research team-including experts from the Charles E. Schmidt College of Science at Florida Atlantic University and the Universities of Barcelona and Pompeu Fabra in Spain, say that they carried out the study to determine whether English- and Spanish-learning infants could perceive people's facial speech gestures and accompanying vocalizations as part of the same event.
According to them, the ability do this is crucial for adaptive communication.
The researchers 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. 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," said Dr. David J. Lewkowicz, professor of psychology in FAU's College of Science, who carried out this research with Drs. Ferran Pons, Salvador Soto-Faraco and Nuria Sebastian-Galles from Spain.
The researchers say that theirs is the first study to provide evidence that perception of audiovisual non-native speech narrows during infancy, precisely during the time that infants are acquiring their native language phonological system.
Their study shows 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.
With a view to testing their hypothesis, the research team 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.
According to the researchers, their test showed that the infants did not prefer one syllable over the other.
The infants were later allowed to hear either the /ba/ or the /va/ syllable for 45 seconds, and were then tested for their preferences for the visual syllables again by showing the silent versions of the side-by-side syllables.
The researchers observed that 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.
The researchers said that the 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.
Besides that, the study also confirmed that the decline in the perception of non-native audiovisual speech persists into adulthood, 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. Rather, it reflects a reorganization of perceptual mechanisms that then leads to decreased sensitivity to non-native sensory inputs," said Lewkowicz.