Not enough distinctions are being made. For better or for worse -- and probably for worse -- let me reiterate a few points that have been raised, because they point to the need for much greater 'conceptual hygiene.' I forget who keeps using the "not enough distinctions" phrase, sounds like the philosopher Jerry Fodor, but I think this point is critical in our current back and forth. Not enough distinctions are being made. Consequently, the discussion that is ongoing about speech is not sufficiently granular.
1. The 'moving parts' (or atoms, or lego blocks, or primitives, or whatever) at the basis of spoken language processing -- both from the input and output sides -- are of course more complex, and larger in number, than we ever discuss here, and consequently the discussion could get hijacked by underspecified concepts. And sometimes is ...
For example ... When we are discussing the so-called motor aspects -- which ones?? To pick up on yesterday's posts, in the Liberman revised motor theory, the objects of perception are intended articulatory gestures. As was rightly pointed out, how close to actual motor output are such objects? That is itself a topic of inquiry, and a complicated one at that. The motor system is not monolithic, and it matters a great deal whether we are working on neuronal populations that form the immediate substrate of motor output or populations that are richly connected to sensory areas but are distal to, say, M1 neurons. Incidentally, the literature on eye movements is worth looking to for some inspiration in this regard. More on that eventually.
Similarly, we should, I think, be very very careful about distinguishing forward models (that rely on a strong predictive element) from the motor generation of output. A forward model is associated with output -- but is not the same as the motor program that generates the output. And, crucially, a forward model does not have to be instantiated in motor cortex. That's an entirel different question as again was pointed out.
If we think of the phrase "motor" as referring to the neural circuitry that underlies output generation (i.e. the part of motor cortex that is required or speaking), then a motor theory is, I think, wrong, and if Luciano Fadiga (Hi Luciano -- thanks for participating!) is intending this view of a motor theory then it won't work.
2. Greg keeps harping on this, and let me also emphasize: what you use as a task matters a great deal. There is, from a perceptual, computational, and neurophysiological point of view a huge difference between, say, syllable discrimination in an experimental task setting, on the one hand, and comprehending spoken sentences, on the other. There are OBVIOUSLY some overlapping component processes, but can we please please please move on from this point? This issue has been rehearsed and discussed since the late 1990s ...
All else being equal, I am persuaded by an auditory view of speech perception, in which internal forward models (but not motor output models) play an important role (for example incorporating algorithms such as analysis-by-synthesis). I am happy to see and admit to a modulatory role of the type demonstrated by Luciano Fadiga and colleagues, but that activity is not epistemologically prior or causally necessary.