Neural Bases of the Motor Theory of Speech Perception Tyler Perrachione HST.722 / 9.044J 11/1/2007 tkp@mit.edu

The Speech Chain(s)

Acoustic Variability and Phonological Invariance Formant trajectories Aspiration following sibilants Coarticulation Intra/inter-speaker variation Syllable position

Bonuses from Motor Theory Parsimonious w/r/t a unified system for production and perception Accounts for mapping between highly variable acoustic signal and invariant phonological percepts Consistent with perception as internal representation of distal events

Why is the Motor Theory Always the Alternative Theory? Lack of direct behavioral evidence Putatively less computationally efficient Less intuitive Skepticism about theories that are “too big” Haskins labs are a bunch of crackpots

New Evidence - fMRI Hearing speech activation overlaps regions of activation during the production of speech (Wilson et al., 2004 Nat. Neurosci.)

New Evidence - fMRI Lip regions of motor cortex more active during perception of /p/, tongue regions more active during perception of /t/ (Pulvermüller et al., 2006, PNAS)

New Evidence - TMS Stimulation of prefrontal cortex impairs speech, not nonspeech, perception (Meister et al. 2007, Current Biology) Dissociable loci of phonological and semantic processing in IFG (Gough et al. 2005, J. Neurosci). Combined PET and TMS to locate speech perception modulation in IFG (Watkins & Paus, 2004, J. Cog. Neuro) Enhanced motor-evoked potential when listening to speech vs nonspeech (Watkins et al. 2003, Neuropsychologia)

Relation to Other Topics Somatic & auditory integration in DCN for spatial location perception => sensory & motor integration in IFG for speech perception Stuttering and production / perception feedback loop Putative human mirror-neuron system Lateralization of speech-language processes

Why Discuss Motor Theory? Sensorimotor integration in perception has implications beyond speech: Conspecific identification, theory of mind, internal world models / simulations Implications for diagnosis and treatment of speech-sound disorders Broader perspective on nature of speech / language integration and evolution