For your Happy New Year reading enjoyment, let me point you to my just (online) published synthesis of computational, psycholinguistic, and neuroanatomic research on speech production: Hickok, G. (2012). Computational Neuroanatomy of Speech Production, Nature Reviews Neuroscience. The aim was to shatter barriers between the motor control folks, the psycholinguists, and the neuroscience oriented researchers studying speech production. This integration has some interesting consequences (in my view). Here are a few:
1. Speech motor control is hierarchically organized (no big surprise) with an auditory-(pre)motor circuit representing a relatively higher level and a somatosensory-motor circuit a relatively lower level.
2. The auditory grounded circuit primarily deals in units the size of syllables whereas what we normally think of as segmental units (~phonemes) are processed primarily in the lower-level somatosensory based circuit. Yes, I'm arguing that "phonological representation" is distributed over auditory and somatosensory cortex.
3. Phonological encoding, in the sense of typical two-stage models of speech production is achieved in the context of a state feedback control circuit (from the motor control tradition).
4. Efference copy signals as they are currently conceptualized in the motor control literature do not exist (let's see what kind of push back I get on this one!). That is, the motor controller does not issue a copy of the command that it has executed. Rather, motor to sensory feedback is part of the motor planning process from the start. In other words, in my view, the "efference copy" is an iterative feedback loop that enables sensory systems to be a part of the programming of the movement rather than just evaluating the outcome of movement commands. This conceptualization integrates the notion of motor planning, efference copies, forward prediction, and error correction into one mechanism. In addition, this computational architecture solves the problem of how both internal and external feedback monitoring can be achieved by the same network even though the timing of the two feedback sources differ. I present a simply simulation to demonstrate the feasibility of these assumptions.
5. Forward predictions are instantiated computationally as inhibitory inputs to sensory systems.
6. Conduction aphasia and apraxia of speech involve disruption to two different components of the same hierarchical level of state feedback control, the relatively higher level auditory-pre-motor circuit.
7. Sensory representations are central to the motor planning process and explain the tight interaction between sensory and motor speech systems. It is a sensory theory of speech production in a sense as opposed to a motor-oriented theory of speech perception.
I would love to get your thoughts on this paper. There's lots in here to discuss/argue about and it will be fun to debate some of the data and/or theoretical claims.