Patterns of cortical interactivity supporting speech production and lexical retrieval: A graph signal processing approach at the individual level

Abstract

Abstract Although seemingly easy, speech production is a complex action involving several processes and brain regions and is often studied through the use of picture naming paradigms. Several brain regions have been identified as being involved in speech production and lexical retrieval, including several areas of the left lateral, ventral, and medial temporal cortex, and of the left lateral and medial prefrontal cortex (PFC). However, little is known about how these regions interact to allow us to produce and retrieve words so efficiently as we speak. In this study, we used graph signal processing, and in particular graph learning, to analyze intracranial electroencephalography data recorded directly at the cortical surface (i.e., electrocorticography, ECoG) in the blocked cyclic picture naming task (recently reported in Ries et al., 2017). In this task, pictures are named several times in semantically homogeneous (HOM) versus heterogeneous (HET) blocks. Lexical retrieval, and in particular our ability to select words from competing alternatives, is hampered in HOM vs. HET blocks with increasing repetitions of the pictures, an effect referred to as the semantic interference effect. The method we used can infer the connectivity between different brain regions from the recorded ECoG signals through an optimization process that considers the entire connectivity map instead of pair-wise methods such as more commonly used correlation measures. Our results show that distant left frontal and temporal brain regions as well as PFC regions are functionally connected in picture naming, and that connection strength is variable between pairs of brain regions and between participants. Semantic interference on naming latencies was not present in all participants and varied in terms of when naming latencies became slower in HOM versus HET blocks. Connection weights were generally sensitive to semantic context in the participants showing a semantic interference effect on behavior but in highly variable ways across participants and in different region pairs. In sum, our results are consistent with the hypotheses that left frontal and temporal regions are functionally connected during picture naming and that intra-frontal connections seem particularly important in this paradigm manipulating lexical retrieval difficulty. Further research is needed to explore the cortical interactions supporting lexical retrieval.