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Segretation vs. Integration of language and inhibition in the inferior frontal cortex

Poster A6 in Poster Session A, Thursday, October 6, 10:15 am - 12:00 pm EDT, Millennium Hall

Cristina Cano Melle¹, Tatiana Davydova¹, Lidón Marin-Marin¹, Esteban Villar-Rodríguez¹, César Ávila¹; ¹Jaume I University

INTRODUCTION: One of the oldest findings in human neuroscience is that the brain control of some cognitive functions is lateralized, that is, they are mainly processed in one of the two cerebral hemispheres. Atypical functional lateralization has been considered a risk factor toward some neurodevelopmental disorders such as schizophrenia or dyslexia, and recent studies have hinted at lower cognitive capacities in some atypical individuals. However, the specific mechanisms behind these relationships are still unclear. Here, we will test the hypothesis that a weak hemispheric distribution of functions is the relevant factor to understand this cognitive dampening, via increasing the interhemispheric transfer. We will focus on the hemispheric specialization present in the inferior frontal cortex (IFC), defined by language production in the left hemisphere and inhibitory control in the right hemisphere. METHOD: We preselected 86 left-handed individuals following an fMRI assessment of their language production (verb generation task) and inhibitory control (stop-signal task). Participants were divided into functionally segregated (n=38) and functionally integrated (n=48). Segregation was defined as having both language and inhibition clearly separated in different hemispheres, whether typically (left language and right inhibition) or atypically (left inhibition and right language). The integrated group comprised individuals that did not show strong hemispheric specialization in at least one of the two functions (Laterality Index ranging from +40 to –40). We compared these two groups in their cognitive performance during response inhibition (reaction time, RT; and stop-signal reaction time, SSRT) and word/pseudoword reading (speed and accuracy), as well as in their preclinical schizotypal traits via the Schizotypal Personality Questionnaire (SPQ). Additionally, we tested if differences existed in their interhemispheric connectivity between both frontal lobes, functionally (resting-state fMRI) and structurally (voxel-based morphometry). RESULTS: The integrated individuals presented a higher SSRT (t83=1.73; one-tailed P=0.04) during the stop-signal task, indicating an inhibition processing 20 msec slower. No differences were found in the RT. When testing their reading skills, although no differences were found in speed, the segregated group presented a higher accuracy (F=9.69; P=0.003) when reading long unfamiliar words. The integrated group scored higher in the SPQ scale (t81=2.03; one-tailed P=0.02). We confirmed that the integrated group presented a higher interhemispheric functional coupling in the pars triangularis of the IFC (t76=2.02; one-tailed P=0.02), and a larger genu volume (t82=1.71; one-tailed P=0.04) than the segregated group. CONCLUSION: We have presented evidence that left-handers lacking hemispheric specialization in either function of the IFC (i.e. integrated distribution) are: (1) less cognitively efficient, (2) more predisposed to certain neurodevelopmental disorders, and (3) more connected interhemispherically. In basis of these results, we propose that hemispheric specialization enables parallel processing of separate cognitive functions and decreases the time delay associated to interhemispheric traffic, acting as a mechanism behind the differences in cognitive performance. Crucially, these connectivity differences would also fit our current knowledge about the typical ontogenesis of the language network, defined by a shift from inter- to intrahemispheric connectivity. In conclusion, our data supports the notion that hemispheric specialization may be phylogenetically and ontogenetically facilitated for a higher cognitive efficiency.

Topic Areas: Control, Selection, and Executive Processes, Language Development/Acquisition