I. Introduction
The so-called symbol grounding problem (SGP) [1]–[3] refers to the way in which a cognitive agent forms an internal and unified representation of an external word referent from the continuous flow of low-level sensorimotor data generated by its interaction with the environment. In this paper, we focus on solving the SGP in the context of human-robot interaction (HRI), where a humanoid iCub robot [4] acquires and expresses knowledge about the world by interacting with a human partner. Solving the SGP is of particular relevance in HRI, where a repertoire of shared symbolic units forms the basis of an efficient linguistic communication channel between the robot and the human.