Neuroimaging of speech motor control in typical development and in Childhood Apraxia of Speech (CAS)

<p dir="ltr">Articulography and functional neuroimaging are two major tools for studying the neurobiology of speech production. Until now, however, it has generally not been feasible to use both in the same experiment setup because of technical incompatibilities between the two methodologies. The present thesis employed a novel methodology -- termed Magnetoencephalography for the Assessment of Speech Kinematics (MASK) -- to obtain kinematic profiles of oro-facial movements during speech, with concurrent and time-registered magnetoencephalographic (MEG) measurements of brain activities associated with these movements. The thesis describes the results of three experiments designed to accomplish the following aims:</p><p dir="ltr"><b>Experiment 1. </b>Using MASK speech tracking, to measure and characterise speech kinematic profiles, and to validate these measurements by comparison to profiles obtained with measurements from a conventional electromagnetic speech tracking setup and the published literature. The findings indicate that the kinematic characteristics (such as amplitude, duration, and velocity) extracted from speech movements recorded with MASK closely mirror several crucial kinematic traits observed in EMA measurements, which have been well-documented in the existing literature.</p><p dir="ltr"><b>Experiment 2. </b>Using MEG neuroimaging, to determine what speech neuromotor control regions are activated with reiterated nonword speech tasks, which are extensively employed in articulographic studies but have rarely been used in the context of functional neuroimaging. Results showed that (1) for the adult participants, the speech task elicited focal clusters of activation centred in the middle portion of the left prefrontal gyrus (midPCG), and immediately adjacent areas of the middle prefrontal gyrus. The midPCG cluster was located immediately inferior to the button press cluster in the hand knob of the precentral gyrus; (2) for the typically developing children speech-elicited midPCG clusters were bilateral rather than left-lateralised; (3) in contrast, children with CAS showed only right-lateralised activation of the midPCG, and a complete absence of speech-related beta-band activity.</p><p dir="ltr"><b>Experiment 3. </b>Using multivoxel pattern analysis (MVPA) techniques, to map MASK-derived movement profiles onto MEG-derived patterns of neural activity. The results show that (1) speech sensorimotor cortex can be reliably located in peri-rolandic regions of the left hemisphere; (2) mu (8-12 Hz) and beta band (13-30 Hz) neuromotor oscillations are present in the speech signals and contain information structures that are independent of those present in higher-frequency broadband noise signals associated with overt speech movements in the MEG scanner; and (3) representational similarity analysis (RSA) provided evidence for weak but significant encoding of the kinematic relationship between stiffness and duration for the bilabial closure gesture. Evidence for kinematic encoding was found only in the beta frequency band, at a latency window starting about 100 ms before the onset of the opening movement of the bilabial closure.</p>