The cortical organization of lexical knowledge: a dual lexicon model of spoken language processing

Abstract

Current accounts of spoken language assume the existence of a lexicon where wordforms are stored and interact during spoken language perception, understanding and production. Despite the theoretical importance of the wordform lexicon, the exact localization and function of the lexicon in the broader context of language use is not well understood. This review draws on evidence from aphasia, functional imaging, neuroanatomy, laboratory phonology and behavioral results to argue for the existence of parallel lexica that facilitate different processes in the dorsal and ventral speech pathways. The dorsal lexicon, localized in the inferior parietal region including the supramarginal gyrus, serves as an interface between phonetic and articulatory representations. The ventral lexicon, localized in the posterior superior temporal sulcus and middle temporal gyrus, serves as an interface between phonetic and semantic representations. In addition to their interface roles, the two lexica contribute to the robustness of speech processing.

Figure 1. Dorsal and ventral lexicons shown within the context of left hemisphere components of a dual stream processing model

Dorsal stream components are shown in red, and ventral components are shown in blue in this two lexicon model. The bilateral posterior superior temporal gyrus (pSTG), shown in green, is the primary site of acoustic-phonetic analyses of unmodified natural speech. More anterior portions of STG may be associated with the mnestic grouping processes. The dorsal lexicon, found in the supramarginal gyrus and parietal operculum (yellow), mediates the mapping between acoustic phonetic structure and the left dominant articulatory network including premotor cortex, posterior IFG and anterior insula described by Hickok and Poeppel (2001; 2004; 2007), with connectivity supplied by divisions II and III of the superior longitudinal fasiculus (SLF II and SLF III). The angular gyrus is hypothesized to play a role in the identification of sublexical units. The ventral lexicon (yellow), localized in the posterior middle temporal gyrus (pMTG) and adjacent tissue mediates the mapping between acoustic-phonetic representations in pSTG and conceptual representations associated with a semantic hub in the temporal pole that integrates aspects of semantic representation associated with a widely distributed conceptual network. The pMTG and temporal poles are connected by the inferior longitudinal fasiculus (ILF). Direct connections between pMTG and the IFG are supplied by the extreme capsule (EmC).