Acquisition of a "mirror" system for speechreading

Articulatory mimicry is a spontaneous feature in both deaf and hearing infants. We discuss the role of this activity in the perception of visual speech, and speculate on how it shapes the underlying neural circuitry. We argue that in the early stages, speechreading involves an active phase of selection and sequencing of motor plans corresponding to representations of visible articulators acquired during articulatory mimicry. This sequencing activity results in activation of lateral and medial premotor areas (BA6) which we observed in our fMRI study of speechreading in naive subjects. As the repertoire of visual-motor associations expands, the automatic recognition of the visual stimulus (and the retrieval of the corresponding motor plan) becomes possible, consistent with the activation of the left inferior frontal gyrus (putative locus of the human mirror system) reported in studies of speechreading of trained stimuli. We conclude by outlining a computational model, and reporting on simple experiments of deferred head imitation.