Exploring semantic grounding in the posterior parietal cortex

This study examines the evolving perspective on semantic processing, which has shifted from the traditional view of an isolated semantic memory system to one that recognizes the involvement of dynamic, distributed neural networks. Recent evidence supports the hypothesis that semantic processing engages both modality-specific and multimodal regions, with the latter serving as integrative "semantic hubs." In this context, our research focuses on the posterior parietal cortices (PPC) and their role in processing space-related semantics. We utilized a low-frequency repetitive Transcranial Magnetic Stimulation (rTMS) protocol targeting the PPC in 11 healthy participants across two tasks. In the first task, in which participants read aloud words from various semantic categories, including space-related terms, no rTMS effects were observed. In the second task, which required participants to respond aloud in a dichotomous manner to questions that either involved or did not involve spatial relations, rTMS over the right PPC impaired response accuracy for both spatial and non-spatial sentences. Additionally, it selectively increased reaction times for spatial sentences, whereas no effect was found for non-spatial sentences. No significant effects emerged following stimulation of the left PPC. The outcomes of the second task, and particularly the increased reaction times when spatial terms were used within sentences to establish spatial relationships, shed light on the multifaceted functions of the right PPC, highlighting its potential role in the semantic processing of space-related words. These findings provide insight into the distributed nature of semantic networks and the specialized contributions of modality-specific areas and multimodal semantic hubs.