A tale of two gradients: differences between the left and right hemispheres predict semantic cognition

Abstract

Decomposition of whole-brain functional connectivity patterns reveals a principal gradient that captures the separation of sensorimotor cortex from heteromodal regions in the default mode network (DMN). Functional homotopy is strongest in sensorimotor areas, and weakest in heteromodal cortices, suggesting there may be differences between the left and right hemispheres (LH/RH) in the principal gradient, especially towards its apex. This study characterised hemispheric differences in the position of large-scale cortical networks along the principal gradient, and their functional significance. We collected resting-state fMRI and semantic, working memory and non-verbal reasoning performance in 175 + healthy volunteers. We then extracted the principal gradient of connectivity for each participant, tested which networks showed significant hemispheric differences on the gradient, and regressed participants’ behavioural efficiency in tasks outside the scanner against interhemispheric gradient differences for each network. LH showed a higher overall principal gradient value, consistent with its role in heteromodal semantic cognition. One frontotemporal control subnetwork was linked to individual differences in semantic cognition: when it was nearer heteromodal DMN on the principal gradient in LH, participants showed more efficient semantic retrieval—and this network also showed a strong hemispheric difference in response to semantic demands but not working memory load in a separate study. In contrast, when a dorsal attention subnetwork was closer to the heteromodal end of the principal gradient in RH, participants showed better visual reasoning. Lateralization of function may reflect differences in connectivity between control and heteromodal regions in LH, and attention and visual regions in RH.

Functional geometry of auditory cortical resting state networks derived from intracranial electrophysiology

“…A recent study that applied DME to RS-fMRI data presented evidence for hemispheric asymmetry in semantic networks, which correlated with semantic task performance 93 . The large dataset in that study and the availability of data from both hemispheres in every participant likely facilitated detection of those differences.…”

Heritability and cross-species comparisons of human cortical functional organization asymmetry

“…Consequently, these regions are placed at different positions along the cortical hierarchy, providing novel insights concerning the system-level variations in the asymmetric brain. Indeed, recent research suggests that the principal gradient is asymmetric ( Liang et al, 2021 ; Gonzalez Alam et al, 2022 ) and that the degree of asymmetry relates to individual differences in semantic performance and visual reasoning ( Gonzalez Alam et al, 2022 ).…”

Mapping lesion, structural disconnection, and functional disconnection to symptoms in semantic aphasia

“…Neuroimaging studies show that bilateral activation underlies executive function (Camilleri et al, 2018;Fedorenko et al, 2013) and (especially in visual-spatial tasks), the most robust responses are often right-lateralised (Dick et al, 2019;Gonzalez Alam et al, 2018). A recent study found that control networks show different patterns of connectivity across the hemispheres, with left-lateralised control regions showing stronger connectivity with heteromodal regions of the default mode network (Gonzalez Alam et al, 2021). In this way, the left lateralised nature of the semantic control network revealed by our analysis may be adaptive, allowing control regions to separate from visual-spatial responses when internal aspects of cognition are constrained.…”

“…While the SCN is largely left-lateralised, the MDN comprises distributed bilateral regions (Camilleri et al, 2018;Gao et al, 2021;Gonzalez Alam et al, 2019). As such, effective semantic control should involve connectivity within the lefthemisphere, while domain-general control should rely more on interhemispheric connectivity (Gonzalez Alam et al, 2021), allowing the integration of information across right-hemisphere regions dominant in the control of visuospatial processing with contralateral frontal regions (Wu et al, 2016). As a result, structural or functional disconnection-symptom mapping may separate semantic control impairment from general executive deficits in a way that cannot be achieved by lesion-symptom mapping in patients with left-hemisphere stroke.…”