A generative model of speech production in Broca’s and Wernicke’s areas

Cathy Price

doi:10.3389/fpsyg.2011.00237

Abstract

Speech production involves the generation of an auditory signal from the articulators and vocal tract. When the intended auditory signal does not match the produced sounds, subsequent articulatory commands can be adjusted to reduce the difference between the intended and produced sounds. This requires an internal model of the intended speech output that can be compared to the produced speech. The aim of this functional imaging study was to identify brain activation related to the internal model of speech production after activation related to vocalization, auditory feedback, and movement in the articulators had been controlled. There were four conditions: silent articulation of speech, non-speech mouth movements, finger tapping, and visual fixation. In the speech conditions, participants produced the mouth movements associated with the words “one” and “three.” We eliminated auditory feedback from the spoken output by instructing participants to articulate these words without producing any sound. The non-speech mouth movement conditions involved lip pursing and tongue protrusions to control for movement in the articulators. The main difference between our speech and non-speech mouth movement conditions is that prior experience producing speech sounds leads to the automatic and covert generation of auditory and phonological associations that may play a role in predicting auditory feedback. We found that, relative to non-speech mouth movements, silent speech activated Broca’s area in the left dorsal pars opercularis and Wernicke’s area in the left posterior superior temporal sulcus. We discuss these results in the context of a generative model of speech production and propose that Broca’s and Wernicke’s areas may be involved in predicting the speech output that follows articulation. These predictions could provide a mechanism by which rapid movement of the articulators is precisely matched to the intended speech outputs during future articulations.

Highlights

Speech production is a complex multistage process that converts conceptual ideas into acoustic signals that can be understood by others
For the current study of speech production there are two advantages of using positron emission tomography (PET) rather than fMRI: the PET scanning environment is quieter for recording the presence or absence of speech output; and the regional cerebral blood flow signals are not distorted by air flow through the articulators
GREATER ACTIVATION FOR SILENT SPEECH THAN NON-SPEECH MOUTH MOVEMENTS There were two areas where activation was significantly higher for silent speech than non-speech mouth movements: the left posterior superior temporal sulcus and the left dorsal pars opercularis within the inferior frontal gyrus extending into the left middle frontal gyrus

Summary

Introduction

Speech production is a complex multistage process that converts conceptual ideas into acoustic signals that can be understood by others. The stages include conceptualization of the intended message, word retrieval, selection of the appropriate morphological forms, sequencing of phonemes, syllables, and words, phonetic encoding of the articulatory plans, initiation, and coordination of sequences of movements in the tongue, lips, and laryngeal muscles that vibrate the vocal tract, and the control of respiration for vowel phonation and prosody. In addition to this feed forward sequence, auditory, and somatosensory processing of the spoken output is fed back to the motor system for online correction of laryngeal and articulatory movements (Levelt et al, 1999; Guenther et al, 2006). The aim of the current study was to identify brain responses related to the internal model and to consider how these responses might predict auditory output prior to auditory or sensorimotor feedback

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Conclusion