Cross-task evidence for language processing recruiting an episodic buffer of working memory located in the visual word form area

Abstract

It has been an excessively expansive practice to consider speech or other mind-external forms such as gestures and written symbols, as equivalent to language, which are only possible externalization forms for human language. Another universally broadcasted notion is that language is communication, a shared trait across species and of course one possible usage of the externalized language.

Rather, recent advances in experimental and theoretical science of language suggest that human language is ‘an instrument of thought’ for yielding complex thought and there should be a sharp distinction between the ‘internalization’ language module and its ‘externalization’. The internalization computations construct internal syntactic and conceptual-intentional representations and these internal linguistic representations are mapped to mind-external ordered output forms, via sending instructions to and receive feedback from externalization brain circuits for perception and production of spoken, signed and written language in humans. In concrete terms, the core language faculty is a biologically determined generative procedure (GP) of syntactic rules that takes lexical items as fundamental units (i.e., word-like conceptual atoms) and implements recursively to generate unbounded hierarchically structured expressions. Each expression will be assigned an interpretation at two critical language-external interfaces: (i) a sensory-motor interface (SM) that generates these structured internal expressions as sequentially ordered forms via acoustic, speech or limb brain ‘externalization’ circuits (e.g. speech sounds or written symbols); and (ii) a conceptual-intentional interface (CI) that generates instructions for intention, inferences, reasoning and semantic meaning.

Importantly, the externalization poses a challenging cognitive problem that how the internal language module as a recently evolved cognitive system is connected to externalization sensory-motor systems that are evolutionarily more ancient and quite independent to language. One question remaining unclear is whether there exists a common mechanism mediating the mapping between the internalization module and externalization of different modalities. Particularly, addressing this question is hard on the single-word level, since the GP does not contain . Under this design, given that externalizing and internalizing lexical items do not purposively generate syntactically structured expressions and thus shall involve the syntactic rules and representations of GP. If there exists a common mechanism mediating the mappings between internal lexical items representations and their external forms of different modalities, I hypothesize that this mechanism is not the GP but rather a workspace assumed to hold retrieved lexical items (as well as any GP constructed units) and interface with the GP, the CI and the SM. Also, this workspace is should to be the episodic buffer of working memory. According to the theoretical nature of the episodic buffer, four hypothetic criteria of brain activity patterns were framed to identify this system.

Here, this thesis conducted four fMRI experiments using paradigms wherein subjects were instructed to perform four different word-level (i.e. lexical-item level) spoken-/written-language comprehension and production tasks. Intriguingly, this study identified a shared workspace across four tasks located in the visual word form area, namely the middle portion of left ventral occipitotemporal area. Accordant with the buffer’s putative role, multiple analyses conjointly suggest that this area should be part of the episodic buffer system of working memory.