Development of infants' communication and expression of emotion through vocalization.

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Infants' ability to express emotin through vocalizations is studied through perceptual rating experiments for voice samples recorded from ten infants at 2,6,9,12 and 17 months of age. Listening subjects were four groups with normal hearing, i.e., students, mothers, care takers, and children. Following results were obtained.(1) The context dependency was different among the listener groups. The students biased their scores depending on the contexts without significant changes in inter-relationships between the rating scales. On the other hand, the mothers changed inter-relationships between the rating scales depending on the contexts. The mothers seemed to change perceptual weights, based on which they would hear a specific aspect of emotion, rather than simply biasing the rating scores under a certain context. Perception of emotion seemed being influenced by differences in listeners' child-rearing experiences. (2) Both the adult and child listeners perceived rich contents of emotions from the voice samples recorded even at 6 month of age. More factors were extracted from the rating scores given by the children than by the adults via factor analyzes. Children are more sensitive in perception of emotions from infants' vocalization than the adults. Children seem to perceive richer emotions from infants'vocalizations than the adults. (3) By a factor analysis for 200 voice samples recorded at 2 months of age, three factors representing emotional contrast of pleased vs. frightened, calm vs. surprising, and speaking vs. singing were extracted. Acoustical analyzes showed that these perceptual factors significantly correlate with the acoustical parameters such as F0 range, F0 variation type, F0 minimum, F0 maximum, vocalization length, and the number of segments.These results suggest that infants even at 2 months of age can produce vocal elements necessary to express emotional contrasts that are identifiable for adult listeners.