When people reflect upon their own inner experience, they often report that it has a verbal quality (Baars, 2003). Also referred to as verbal thinking, inner speaking, covert self-talk, internal monologue, and internal dialogue, inner speech has been proposed to have an important role in the self-regulation of cognition and behavior in both childhood and adulthood,

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Despite its apparent importance for human cognition, inner speech has received relatively little attention from psychologists and cognitive neuroscientists, partly due to methodological problems involved in its study."

Source : Inner Speech: Development, Cognitive Functions, Phenomenology, and Neurobiology Psychological Bulletin, 2015 Volume 141, Issue 5

After more than a century of study, Inner Speech remains paradoxical - with little agreement among psychologists as to its exact function, importance, and evolutionary pathways.

The Laboratoire de Psychologie et NeuroCognition, at the University of Grenoble, France, has a research lab dedicated to investigating the phenomenon.

Note: Philosophers have also focused on the implications of Inner Speech : See : Why are you talking to yourself? The epistemic role of inner speech in reasoning Noûs, Early View. June 2021.