Infants' categorization of intransitive human actions

How infants form categories of actions is a crucial component of both their cognitive and linguistic development. Actions create the dynamic world that infants live in and verbs, which encode actions, are the architectural centerpiece of language. This research investigates how infants form nonlinguistic categories of natural human actions when they begin to learn verbs, how infants' language development relates to and interacts with their action categorization, and how a common label impacts action categorization. Study 1 demonstrated that 10- to 12-month-old English-learning infants can form nonlinguistic categories of hopping and marching performed by five actors along five different paths. Studies 2A and 2B established that infants of the same age can discriminate between the within-category action exemplars. Studies 3A and 3B explored a possible explanation for infants' stronger performance in forming a category of marching than hopping. Study 4 engaged 19- to 21-month-olds in the same nonlinguistic categorization task to examine possible interactions between categorization and language development, as a major milestone in language development -- the vocabulary spurt -- typically occurs around 18 months. Results showed that the older infants did not show successful categorization overall, but the infants whose relational vocabulary scores (number of verbs and prepositions produced) were below the median performed more similarly to the 10- to 12-month-olds. Finally, Study 5 examined whether labeling the actions with a novel verb could facilitate 19- to 21-month-olds' categorization. These studies suggest that (1) at the end of the first year infants possess the cognitive foundation for learning verbs, (2) when rapid word learning begins infants become aware of the role of language in categorization and can be guided by language in forming action categories, and (3) action categorization and the acquisition of relational vocabulary are intimately linked.