Ok, first of all, I shouldn't be working on a weekend. That's David's job, but here I am anyway...
By leaving the issue out of my previous post, I was hoping someone would raise a question about whether motor-related information CAN influence speech perception, and how that might be explained on an "acoustic" theory of speech perception. Without even encouraging him, one of my own students brought it up. (I swear it wasn't a plant.) Kenny Vaden made the following point in a comment on my last post:
"While aphasias undermine the MT [Motor Theory] claim that phonological representations are *completely* motoric/gestural, the survival of speech perception without production does not mean that motoric information is not available to speech perception at all."
He's absolutely right. Knowledge of how speech is produced does appear to influence our perception of speech, at least under some circumstances. The paper we read discusses several lines of evidence in support of this position, and the data are reasonably compelling. The most obvious demonstration of this is probably the McGurk effect, but there are others.
So we must acknowledge that the motor knowledge can influence perception. But does this mean that an acoustic model is not correct? Do we have to admit that at least part of speech perception, or speech perception under some circumstances, involves perceiving gestures? No, we don't. Here's a simple explanation: knowledge of how speech is produced can have a top-down influence on the acoustic perception of speech information. Top-down expectations of a variety of sorts can influence all kinds of perceptual events, including speech. For example, the lexical status of a CVC syllable affects the perception of its constituent phonemes (e.g., category boundaries shift toward the lexical item in a b-p continuum with bag and pag as endpoints). So why can't motor expectations (e.g., forward modeling, predictive coding, analysis by synthesis -- whatever you want to call it) have a top down influence on acoustic representations? They can.
Conclusion: motor effects on perception do not falsify an acoustic theory of speech perception. But they do suggest that motor knowledge can influence perception in a top-down fashion, just like many types of knowledge can.