The neural organization of speech production has been a topic of intense investigation even before Broca’s namesake area became an early focus and much has been learned in the century and a half since. We now understand that speech production involves a complex, hierarchically organized network involving many areas including primary orofacial motor cortex, lateral and medial premotor cortex, auditory cortex, somatosensory cortex, and subcortical systems (Guenther, 2016), all of which integrate with higher-order linguistic systems in the frontal, temporal, and parietal lobes involved in phonological and morphosyntactic processing (Hagoort and Indefrey, 2014; Hickok and Poeppel, 2007; Indefrey, 2011).
Our focus here is on a subcomponent of this broader system involved in what we will call speech coordination, which we define as a level between the execution of motor commands at the lowest cortical level in primary motor cortex and higher-order linguistic levels of speech production planning, such as accessing syllabic codes (Levelt and Wheeldon, 1994), syllable sequencing (Peeva et al., 2010; Rong et al., 2018), or morphosyntactic processing (Lee et al., 2018; Matchin and Hickok, 2020). This situates our neuroanatomical focus roughly in premotor cortex and our computational focus on a system that coordinates movements of a wide range of speech articulators, from lips to larynx. Deficits at this level of the motor speech system, we believe, yield clinical syndromes such as apraxia of speech or stuttering as opposed to lower-level dysarthrias or higher-level aphasias.
Our primary goal is to test the speech coordination system.
The primary hypothesis we propose to test is that the speech coordination system is composed of (at least) two separable but interacting subsystems, (i) a dorsolateral system coordinating pitch-related aspects speech, prosody, and song and (ii) a ventrolateral system coordinating phonetic articulation at the segment and syllable level. These systems, we propose, are part of parallel hierarchies for speech and language planning, which means that our proposed dual coordination model has implications for the organization of language systems more generally. Some of our proposed experiments will probe this broader system as well as provide explicit tests of our primary hypothesis.
Our hypotheses will be tested using fMRI experiments. The proposed studies are part of a larger programmatic effort to understand the neural organization of this system using converging methods: fMRI on stroke, primary progressive aphasia, and intracranial recordings/stimulation.
