Search Abstracts | Symposia | Slide Sessions | Poster Sessions
Network‑level effects of left and right IFG perturbation during phonological processing: the role of hemispheric dominance
Poster Session B, Wednesday, September 30, 4:30 - 6:30 pm, Wangari Maathai
Robin Gerrits1, Stephanie Forkel2, Gesa Hartwigsen1; 1Research Group Cognition & Plasticity, Max Planck Institute for Human Cognitive and Brain Sciences, 2Research Group Clinical Neuroanatomy of Language, Donders Institute for Brain, Cognition, and Behavior
Asymmetry is a defining feature of the language system. Although many language tasks engage a predominantly left‑lateralized network , there is substantial variability in both the degree and direction of this asymmetry. Such variability may influence the system’s susceptibility to focal perturbations as well as its capacity for recovery following those perturbations. The present study investigated how hemispheric dominance modulates short-term behavioral and neural responses to TMS-induced perturbations of the left and right inferior frontal gyrus (IFG) during a pseudoword rhyming task. Participants (n=36) completed four sessions on separate days. During the first, MRI-only session, they completed a pseudoword rhyming task in which they judged whether pseudoword pairs rhymed (Rhyming condition) or symbol strings matched (Control condition). The fMRI data from this session were used to quantify participant-specific hemispheric dominance and to prepare neuronavigation for the three subsequent TMS-MRI sessions. During these sessions, participants received sham stimulation, 1 Hz rTMS to the left IFG, or 1 Hz rTMS to the right IFG for 30 minutes (counterbalanced between participants), after which they repeated the task in the scanner. This design ensured that each participant’s dominant and non-dominant IFG were stimulated, regardless of language dominance. Left-handers (n=19) were oversampled to boost variability. Effective TMS did not alter group-level behavioral performance or activity at the target relative to sham. Mass-univariate analyses revealed two remote clusters of reduced activation following left IFG stimulation: one centered on the right supramarginal gyrus extending to the superior frontal and (para)cingulate gyrus, and another centered on the right IFG extending into bilateral insula. In contrast, right IFG stimulation upregulated activity in the bilateral frontal pole, as well as the right cerebellum (centered on lobule VIII) and the left lateral temporal lobe (peaking in inferior temporal gyrus). Sham-controlled activation changes in these clusters was not associated with behavioral performance. However, greater activity at the right IFG target itself was associated with reduced accuracy and speed. Individual hemispheric dominance was not related to sham-controlled activity at the IFG targets. After left-sided TMS, more left lateralized individuals showed increased activation in language-unrelated areas, including regions of the default mode, visual, somatosensory and fronto-parietal control network. This activity was associated with faster responses. After right IFG stimulation, more left lateralized individuals showed reduced activation in the inferior parietal lobe and left posterior middle temporal lobe, and this effect was unrelated to performance. Our study revealed that short term neural responses to focal IFG perturbations during phonological processing are expressed mainly remotely rather than at the target itself. Although TMS did not disrupt group level performance, it produced distinct target-depended patterns of remote activation. Importantly, individual differences in hemispheric dominance modulated these effects. The greater engagement of domain-general networks following left IFG stimulation in more strongly left-lateralized participants may reflect a compensatory adjustment supporting task performance. Our results point to hemispheric dominance as an important source of variability in how the language system responds to transient disruptions, and highlight the need to account for individual variability in models of language-network resilience and recovery.
Topic Areas: Phonology,