Temporal dynamics and network mechanisms of articulatory feature encoding during speech production.

Project Summary
The overall goal of this fellowship is to provide excellent training in representational analysis, intracranial
neurophysiology, and network level dynamics by exploring articulatory feature encoding in ventral
sensorimotor cortex (vSMC). The vSMC is the primary cortical region responsible for controlling the precise
articulatory movements required for fluent speech. Despite its well-established role in speech production, it is
not clear when it is engaged relative to earlier stages of processing in speech (lexical access) and overt response
execution, and how its motor plans are shaped by upstream language areas. This project addresses two core
hypotheses regarding the timing and input dynamics of articulatory feature encoding in vSMC. In Aim 1, we
test when articulatory feature encoding, or the word-specific motor plan for production, emerges in vSMC.
Using stereo-EEG (SEEG) recordings in a delayed naming paradigm, patients will prepare to name pictures but
will only speak after a variable delay (0, 400, or 800 ms). This temporally dissociates `early' lexical access from
`late' motor execution, allowing us to determine whether articulatory representations in vSMC are time-locked
to stimulus presentation or the initiation of speech. If encoding is linked to lexical acesss, representational
structure should appear at a consistent post-stimulus time across delays. If it is tied to the go-cue, we expect a
[delay × onset time] interaction in stimulus-locked analyses, and consistent timing in go-cue-locked analyses.
These results will clarify whether articulatory plans in vSMC are accessed automatically with lexical access or
gated by response initiation. In Aim 2, we investigate how upstream language regions—specifically the
posterior middle temporal gyrus (pMTG), angular gyrus, inferior frontal gyrus (IFG), and supplementary motor
area (SMA)—contribute functional input to vSMC. Using event-related causality and high-definition fiber
tractography (HDFT) to identify structurally connected contacts, we will test whether input from pMTG and
angular gyrus is time-locked to lexical access (stimulus onset), while input from IFG and SMA is time-locked to
speech initiation (go-cue). This predicts a [delay × onset time] interaction in stimulus-locked analyses for IFG
and SMA, but not pMTG and angular gyrus. Secondary analyses will examine whether psycholinguistic
complexity (e.g., word frequency, phoneme length) modulate the strength of causal input from upstream
regions, providing insight into whether these inputs are modulated by content-specific information. By
advancing our understanding of how linguistic units are mapped onto motor plans, this work has the potential
to constrain language processing models, to improve functional neurosurgical mapping, and inform the
development of brain-computer interfaces for communication in patients with primary cortical aphasias.
Through advanced training in analytic and clinical methods under expert mentorship, this project is well placed
to prepare me for the next steps of my training, and sets an important foundation for a career as a clinician-
scientist at the intersection of speech neuroscience and translational care.

Grant Number: 1F30DC023481-01
NIH Institute/Center: NIH
Principal Investigator: Jawaad Belkhir