Rhythmic behavioral responses to stimuli from another person are expressed at 18 months of age

The Kyoto University has clarified that rhythmic behavior develops earlier than conventionally thought. Rhythmic behavior is associated with the strength of the social relationship between the subject and the person providing the stimulus.

When listening to good music, do you notice yourself matching your feet or hands with the rhythm without deliberately doing it? Rhythmic behavior that matches another individual is observed in many animals. However, rhythm-synchronized behavioral patterns are unstable and uncertain, such as pant-hooting (behavior in which nearby individuals speak together). Humans, on the other hand, can perform fast, accurate, and sustained rhythm synchronization with each other. When do we acquire the ability to match rhythm with another person, and under what conditions do we perform it efficiently?

The research team of Lira Yu, an assistant professor at the Kyoto University Wildlife Research Center (currently University of Tokyo Graduate School and College of Arts and Sciences JSPS PD), and Masako Myowa, an assistant professor at the Kyoto University Graduate School of Education, attempted to answer this question and conducted an experiment involving 18-, 30-, and 42-month-old children who played a drum in the same rhythm as that of another person. From approximately 18 months of age, children tried to match rhythm with another person when it was easy, and from 30 months of age, matching rhythms became flexible and accurate. The behavior of matching rhythm with that of another person is related to the strength of social ties between the two individuals. Yu stated, "In the future, I would like to examine the budding behavior of babies to acquire the ability to match rhythms with another baby and the relationship between this ability and the strength of the social tie with the caregiver for babies aged 18 months or above." This paper was published online on April 30 in Infancy.

Previous studies have shown that from the age of 30 months, the behavior of beating a drum in the same rhythm as that of another person can be observed. Therefore, the research team focused on the period from 18 to 42 months of age and further from 30 months to 1 year. At 18 months of age, there is a remarkable development of species-specific motor functions and sensory/cognitive abilities, such as bipedal walking and language acquisition. In the pediatric research database at the Kyoto University Graduate School of Education, 66 children aged 18, 30, and 42 months had participated, and the data of 54 participants was analyzed.

In the experiment, two types of partner (mother/robot) beat drums in two types of rhythms (400 or 600 ms) and reactions of the participating children (beating the drums themselves) were recorded with a video camera. In addition, a vibration sensor was attached to the back of the drum to automatically record the rhythm of the participating children beating the drum. As a result, the behavior of controlling one's own drum-beating in response to the partner's fast rhythm (400-ms interval) was seen at the age of 18 months and that of controlling the beat according to the opponent's slow rhythm (600-ms interval) was observed after 30 months of age. In addition, the ability to flexibly and accurately adjust to the rhythm of another person appeared prominently after 30 months of age. In some children at 18 months of age, this behavior was confirmed for slow rhythms only when the mother was the partner. In other words, if one's physical ability allows for the performance of the rhythm, they can match rhythms with another person at an age younger than previously thought (18 months). At this age, the strength of social ties is a factor; the rhythm was only matched when the partner is the participant's mother. Myowa said, "Children with ASD are known to have poor synchronization abilities. Therefore, it has been said that the developmental support program is effective when communicating with the mother, and our result supports this notion. In the future, I aim to clarify how early synchronization abnormalities occur and improve the support program."