This paper presents an initial proposal for a human-centered cognition model intended to support the development and operation of Human Digital Twins (HDTs). Acknowledging the complexity and incomplete understanding of human cognition, this work adopts a systemic and interdisciplinary approach, integrating perspectives from cognitive sciences, social sciences, engineering, and cognitive informatics. The model aims to formalize the representation of human cognitive dimensions through a structured framework of domains, abilities, and processes, with particular attention to sensation, perception, memory, attention, motor skills, language, metacognition, and higher cognitive functions. Beyond offering a descriptive foundation, the model aspires to facilitate the integration of heterogeneous models and data sources into HDTs, thereby supporting more interpretable, adaptive, and context-aware digital representations of human behavior. While this work does not claim exhaustiveness, it seeks to contribute to ongoing efforts toward a more rigorous and interoperable conceptualization of human cognition within digital twin systems.