Dr. Joseph Perkell


Speech Motor Control Group
Speech Communication Group
Research Laboratory of Electronics
Massachusetts Institute of Technology

For an overview of research by the Speech Motor Control Group, see  Movement goals and feedback and feedforward control mechanisms in speech production.

THEORETICAL OVERVIEW: Speech production is arguably the most complicated motor act performed by any species. It engages intricately coordinated actions of the respiratory system, the larynx and the vocal tract to convert discretely specified sequences of words and sounds into a quasi-continuous series of movements and an acoustic output. Our research focus is on part of this process, namely, the control of the phonemic (sound-by-sound) movements of the articulators in the vocal tract. Our approach is based on a theory in which the goals of such movements are specified in sensory (auditory and somatosensory) spaces, and the movements to achieve sequences of these goals are planned by a neurological model of relationships among motor commands and their sensory consequences. This internal model is acquired and maintained with the use of auditory feedback, and it is shaped to some extent by peripheral constraints such as anatomical and biomechanical properties of the articulators. This view provides the theoretical basis for both of our current NIH-sponsored research projects, “Constraints and Strategies in Speech Production” and “Effects of Hearing Status on Adult Speech Production.”

CONSTRAINTS AND STRATEGIES IN SPEECH PRODUCTION: In the Constraints and Strategies project we use a computational neural control model (a simulation of the internal model, developed by our collaborator, Prof. Frank Guenther, of the Dept. of Cognitive and Neural Systems, B.U.) coupled to a biomechanical simulation of the vocal tract to produce articulator movements and a speech output. The vocal-tract model is custom-adapted to the anatomy of individual subjects, on whom we run experiments to collect several kinds of data, including: morphology (from structural MRI scans), speech acoustics, articulatory movements, air flows and pressures, articulator contact and contact forces and electromyographic measures of muscle activity. One set of experiments is designed to refine the combined models and explore control mechanisms; other experiments test hypotheses based on the functionality of the model. As an example, we have recently completed an extensive study on 19 subjects in which we made measures of their speech production and perception and found that speakers with more acute auditory perception produced speech sounds with greater contrast. This result is compatible with the way the neural control model functions and lends support to an important feature of the model – the premise that movement goals are in sensory domains.

EFFECTS OF HEARING STATUS ON ADULT SPEECH PRODUCTION: In the Hearing Status project, we measure changes in speech production that occur in response to changes in hearing. These studies are focused on three populations of adults. The first are participants with normal hearing. Second, are those who learned to speak, then lost hearing and finally regained some hearing with a cochlear implant. Third and last are patients with bilateral acoustic neuromas, a few of whom lost their hearing during the course of the project (This work was funded by a separate NIH grant from 1991 to 1995.) As stated above, we hypothesize that the internal model controlling speech production is acquired in childhood with the use of auditory feedback. In adulthood, it is used to program speech movements essentially “open loop” – that is, without the speaker being influenced by the sound of his or her own voice. However such auditory feedback does come into play to make adjustments in the internal model that are necessitated by changes such as growth of the vocal tract or receiving dentures. Our studies have shown that speech intelligibility remains remarkably intact in postlingually deafened adults, even decades after hearing loss – consistent with open-loop control. On the other hand, we have also shown that speech does deteriorate somewhat with hearing loss, and acquisition of some hearing from a cochlear implant usually leads to some normalization of speech parameters, including measures of speech respiration, vowel and consonant spectra and voicing onset time. These improvements are most observable in those cochlear implant users who show improvements over time in speech perception.