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How do we perceive inner speech? Does it follow the same neural pathways as normal acoustic speech? If yes, what is the extent of overlap between the two neural pathways?

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No, inner speech does not follow the same neural pathway as speech coming in from outside.

Rather, inner speech uses the same neural mechanism as outer speech - that is, speech going out. The neural mechanisms of inner speech can be studied using recently developed technologies such as fMRI imaging of subjects instructed to or prevented from engaging in self-reflection and internal monologue. Such studies suggest that inner speech is primarily localized to Broca's area (LIFG, on the left side of the brain), just as is outer speech (McGuire, 1996; Morin & Hamper, 2012). These findings are consistent with other evidence that inner speech is a form of silent (simulated) outer speech, and thus uses the same physical mechanism. This is based on private speech theories originating in the 1920s and 30s by Jean Piaget and Lev Vygotsky, that inner speech is actually outer speech that children learn to internalize.

                                                   enter image description here

However, if inner speech uses the same mechanism as outer speech rather than the incoming auditory system, then how is it that we "hear" it? One proposal is based on the attenuation mechanism involved in normal speech production. This mechanism filters self-generated sounds from other sounds. It is believed to work analogously to noise cancellation technologies - by predicting self-generated sound and modulating audio processing accordingly. Experiments by Scott (2013) and Scott et al (2013) demonstrate that this attenuation is also active in inner speech, and suggests that perhaps what we "hear" is the prediction of our own voice by this system.

Another approach by Alderson-Day et al (2016) distinguishes between multiple kinds of inner speech - monologic inner speech of the type traditionally studied as above, and dialogic inner speech that adds a second voice (an interlocutor). Brain scans of subjects engaging in dialogic inner speech show additional activity in areas associated with auditory hallucinations, and may therefore be "heard" in a similar way. This is still a very active area of research, so stay tuned!

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    $\begingroup$ As far as I understand, Brocha area is for speech production, which is also used for production of inner speech. But, how do we hear/understand inner speech? $\endgroup$ – akm Jun 26 '16 at 19:22
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    $\begingroup$ There is no need to hear and understand because we already know. Just as you do not need to listen to your outer speech to understand what you are saying. $\endgroup$ – user24582 Jun 26 '16 at 19:52
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    $\begingroup$ Interesting. I wonder if this would help explain the following fact in the Wikipedia article on "SignWriting": "Words may be written from the point of view of the signer or the viewer. However, almost all publications use the point of view of the signer [...]. Sutton originally designed the script to be written [...] from the point of view of the observer, but later changed it to [...] from the point of view of the signer, to conform to the wishes of Deaf writers." $\endgroup$ – sumelic Jun 26 '16 at 21:42
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    $\begingroup$ @user24582: Much literature in cognitive science portrays inner speech as a "rationalizer" or "interpreter" rather than genuine insight - ie, there is no interface between our thought process and consciousness, they seem to exist somewhat separately, with one "guessing" what the other is doing. But this is a large topic for another question. $\endgroup$ – Arnon Weinberg Jun 27 '16 at 16:41
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    $\begingroup$ A related note, Noam Chomsky has suggested that language first developed as an internal mechanism, which facilitated cognition. He further suggests that the relatively sudden appearance of speech as communication indicates that our ability to audibly express and perceive language came later. Therefore, the question might be reversed, "How does our inner speech become expressed through outward, audible communication?" $\endgroup$ – John Yetter Aug 5 '16 at 21:27

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