system
Given the following post, respond as if you're commenting on Reddit:
human
Why can we talk in our heads?

Hey guys, I've always wondered how we are able to talk in our heads. I can say a whole sentence in my head and when I think about that it seems crazy that we can do that. So how are we able to speak in our head without saying it?
gpt
Language related information in the brain is represented at different levels of abstraction. At one end of the spectrum you have the basic visual and/or auditory input coming in from our sensory organs. This information must be preprocessed and analyzed by sensory cortex to reach the point at which we represent it as an actual word form. At the next level, word forms are represented amodally (i.e. equivalently across sensory modalities) and are linked to their grammatical properties. Finally you reach the other end of the spectrum of abstraction where words are linked to their semantic content.

In language production this process is essentially reversed, the primary difference being the fact that the lowest level of abstraction is motor programming of the mouth and throat rather than input from the eyes and ears. Inner speech essentially just stops short of this lowest level - auditory word forms and their grammar are represented, but we don't actually send the necessary information to enunciate them.

It's worth pointing out that not all of our thoughts - even complex, abstract ones - are "spoken" to ourselves in this way. Mental imagery is a good counterexample.

As to why, in an ultimate sense, we have/make use of this ability: from an evolutionary perspective it may simply be a spillover benefit from language (which of course is hugely adaptive for us). However, given the role of language in enhancing working memory via the phonological loop, it may also give us the capacity to think about more at the same time.