The idea that the CNS uses internal models of sensory expectations in motor control is an exciting idea that has captured the imagination of many researchers (including myself). A notable exception is David Ostry, who plays the role of killjoy in this field. David and his lab have been waging a war against internal model “hysteria” – especially in speech motor control – for years, doing careful experiments whose results whose results are challenges for theories of motor control based on internal models to explain. Tomorrow, at Sensorimotor Journal Club, I will present the latest of the efforts of his lab on this front: a study showing no generalization of adaptation to altered jaw dynamics, even from the voiced to the silently articulated production of the same word!:
Tremblay, S., Houle, G., & Ostry, D. J. (2008). Specificity of speech motor learning. Journal of Neuroscience, 28(10), 2426-2434. (link to pdf of article)
The idea that the brain controls movement using a neural representation of limb dynamics has been a dominant hypothesis in motor control research for well over a decade. Speech movements offer an unusual opportunity to test this proposal by means of an examination of transfer of learning between utterances that are to varying degrees matched on kinematics. If speech learning results in a generalizable dynamics representation, then, at the least, learning should transfer when similar movements are embedded in phonetically distinct utterances. We tested this idea using three different pairs of training and transfer utterances that substantially overlap kinematically. We find that, with these stimuli, speech learning is highly contextually sensitive and fails to transfer even to utterances that involve very similar movements. Speech learning appears to be extremely local, and the specificity of learning is incompatible with the idea that speech control involves a generalized dynamics representation.