["Developmental Science, Volume 29, Issue 4, July 2026. ", "\nABSTRACT\n\nThe prosodic characteristics of a native language greatly influence early language acquisition. Yet, Japanese mothers are known to use a specific prosodic structure in infant‐directed vocabulary (IDV)—specifically, three‐mora, two‐syllable words with a heavy‐light pattern—which, crucially, differs from the standard prosodic rhythm of adult vocabulary. This study used near‐infrared spectroscopy to examine hemodynamic responses to the Japanese IDV form in 5‐month‐old (n = 31) and 9‐month‐old (n = 34) Japanese infants, targeting the period before and during the emergence of this preference. The results revealed that oxygenated hemoglobin was greater for the IDV form than for the non‐IDV form in the left superior temporal gyrus (STG) for both age groups, consistent with the advantage of the IDV form observed in previous behavioral studies. Furthermore, this effect was localized to the left middle and left posterior STG in 5‐ and 9‐month‐old infants, respectively, highlighting early sensitivity to its prosodic structure followed by the emergence of phonological representation. This cortical shift, along with an observed trend toward adult‐like patterns, may suggest a broader transition from perceptually accessible IDV structures to the more diverse patterns of standard adult vocabulary. Although 5‐month‐old infants who have not yet exhibited a preference for the IDV form may not have developed specific phonological representations, their brains' ability to process its prosodic pattern could serve as a foundation for subsequent learning. These findings demonstrate that the specific structure of Japanese IDV acts as a foundational scaffold, guiding the transition from initial prosodic tuning to mature word‐level processing.\n\n\nSummary\n\nEarly sensitivity to prosodic structure was observed in 5‐month‐old Japanese infants.\nEmerging phonological representation was observed in 9‐month‐old Japanese infants.\nJapanese infant‐directed vocabulary form serves as a prosodic template.\n\n\n"]