The development and evolution of inner speech: Hippocampal contribution

• Inner speech is a fundamental component of the human mind. • Inner speech involves a conjunction of language and memory networks. • Hippocampal memory replay mechanisms conflate with phonological working memory, yielding inner speech. • In evolution and development of human speech, inner speech provides an internal narrative key for consciousness and social life. Inner speech (IS) is a mental process by which we can execute speech covertly, arranging and sequencing thoughts. While the emergence of IS in the child has been related to the maturation of verbal working memory circuits, the most salient features of this phenomenon, like its spontaneity, repetitiveness, imaginative intensity and especially its episodic contents, remain unresolved. To fill this gap, we propose that IS also relies on a more fundamental cognitive mechanism, which is the function of episodic memory replay, dependent on the hippocampus, in which past experiences are re-instantiated at rest. If confirmed, this would imply that IS arises in development through the maturation of functional connectivity between verbal working memory (vWM) and episodic memory circuits. Furthermore, we predict that events of episodic replay coexist with events of IS in the adult brain. This hypothesis specifies new research lines oriented to assess the role of hippocampal episodic memory in the generation and alterations of IS.