The Functional Connectome of Speech Control

Stefan Fuertinger, Barry Horwitz, Kristina Simonyan, Stefan Fuertinger, Barry Horwitz, Kristina Simonyan

Abstract

In the past few years, several studies have been directed to understanding the complexity of functional interactions between different brain regions during various human behaviors. Among these, neuroimaging research installed the notion that speech and language require an orchestration of brain regions for comprehension, planning, and integration of a heard sound with a spoken word. However, these studies have been largely limited to mapping the neural correlates of separate speech elements and examining distinct cortical or subcortical circuits involved in different aspects of speech control. As a result, the complexity of the brain network machinery controlling speech and language remained largely unknown. Using graph theoretical analysis of functional MRI (fMRI) data in healthy subjects, we quantified the large-scale speech network topology by constructing functional brain networks of increasing hierarchy from the resting state to motor output of meaningless syllables to complex production of real-life speech as well as compared to non-speech-related sequential finger tapping and pure tone discrimination networks. We identified a segregated network of highly connected local neural communities (hubs) in the primary sensorimotor and parietal regions, which formed a commonly shared core hub network across the examined conditions, with the left area 4p playing an important role in speech network organization. These sensorimotor core hubs exhibited features of flexible hubs based on their participation in several functional domains across different networks and ability to adaptively switch long-range functional connectivity depending on task content, resulting in a distinct community structure of each examined network. Specifically, compared to other tasks, speech production was characterized by the formation of six distinct neural communities with specialized recruitment of the prefrontal cortex, insula, putamen, and thalamus, which collectively forged the formation of the functional speech connectome. In addition, the observed capacity of the primary sensorimotor cortex to exhibit operational heterogeneity challenged the established concept of unimodality of this region.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Topology of shared high-degree and…
Fig 1. Topology of shared high-degree and high-strength hubs in the group-averaged RSN and SPN at the minimum density of 60%.
(I) Bar charts show strength values of the top 30% strongest nodes (normalized si ranges 0.9–1.0 and 0.7–0.89 in Fig 4) in both the RSN and SPN. Blue bars highlight nodes that are strength hubs in both the RSN and SPN. (II) Bar charts of the same format show shared degree hubs of the RSN and SPN among the top 30% most interconnected nodes (normalized ki ranges 0.9–1.0 and 0.7–0.89 in Fig 5). (III) The panel demonstrated the strength and degree of hubs shared by the RSN and SPN. All hubs showed a pronounced increase in strength (sum of connected edge weights) from the resting state to speech production (a shift from blue to red connections) with only a moderate, if at all, increase in degree (number of connections). The table provides quantitative measures of nodal values of strength and degree (in parenthesis). Bold numbers indicate that a node is a hub with respect to the respective metric. The 3-D graphs were rendered using Mayavi [57]. Abbreviations: 1 = area 1; 3a = area 3a; 4a/4p = anterior/posterior part of area 4; 5M = area 5M; 6 = area 6; 7A = area 7A; THp = parietal part of the thalamus. The corresponding data are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873
Fig 2. Nodal strength of the group-averaged…
Fig 2. Nodal strength of the group-averaged networks at the minimum density of 60%.
(I,II,III) In the 3-D network visualizations, edge color represents link weight, nodal color corresponds to normalized strength, and nodal size illustrates degree. (IA–E, IIA–E, IIIA–E) Nodes that were removed by the employed elimination strategy are shown in gray. In all networks, strength was normalized to the interval [0 (dark blue)– 1 (dark red)] and split up into four distinct ranges; only nodes with normalized strength in the respective ranges are color-coded. (IV) Bar charts illustrate the proportion of nodes in the respective strength intervals relative to the total number of nodes in the networks. Numbers at the bottom of the bars are node counts for the R = resting state, S = speech production, and Syl = syllable production networks in the corresponding intervals. The 3-D networks were visualized with the BrainNet Viewer (http://www.nitrc.org/projects/bnv/). Abbreviations: 1 = area 1; 3a/3b = areas 3a/3b; 44 = area 44; 45 = area 45; 4a/4p = anterior/posterior part of area 4; 5Ci = area 5Ci; 5M = area 5M; 6 = area 6; 7A/7P = area 7A/7P; 18 = area 18; AmCM = subdivision CM of the amygdala; Cbl-V/VIv/IX/IXv = cerebellar lobules V/VIv/IX/IXv; Cd = caudate nucleus; Cu = cuneus; FG = fusiform gyrus; hIP1-3 = areas hIP1-3; HippFD/SUB = hippocampal subdivisions FD/SUB; hOC4v/hOC5v = ventral parts of areas hOC4/hOC5; IL = insula; IOG/MOG/SOG = inferior/middle/superior occipital gyrus; LG = lingual gyrus; MCC = middle cingulate cortex; MTG = middle temporal gyrus; OP1-4 = operculum; PF/PFm/PFop/PFt/PGa/PGp = areas PF/PFm/PFop/PFt/PGa/PGp in the inferior parietal cortex; SMG/MFG/mFG = superior/middle/medial frontal gyrus; TE1.1–3.0 = areas TE1.1–3.0; THp = parietal part of the thalamus. All connectivity matrices are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873; the codes used to transform the fMRI data to networks can be found at http://research.mssm.edu/simonyanlab/analytical-tools/.
Fig 3. Nodal degree of the group…
Fig 3. Nodal degree of the group averaged networks at the minimum density of 60%.
(I,II,III) In the 3-D network visualizations, both nodal color and size illustrate (normalized) degree. (IA–E, IIA–E, IIIA-E) Nodes that were removed by the employed elimination strategy are shown in gray. In all networks, degree was normalized to the interval [0 (dark blue) – 1 (dark red)] and split up into four distinct ranges; only nodes with normalized degree in the respective range are color-coded. (IV) Bar charts illustrate the proportion of nodes in the respective degree intervals relative to the total number of nodes in the networks. Numbers at the bottom of the bars are node counts for the R = resting state, S = speech production, and Syl = syllable production networks in the corresponding intervals. The 3-D networks were visualized with the BrainNet Viewer (http://www.nitrc.org/projects/bnv/). Abbreviations: 1 = area 1; 3a/3b = areas 3a/3b; 44 = area 44; 45 = area 45; 4a/4p = anterior/posterior part of area 4; 5Ci = area 5Ci; 5M = area 5M; 6 = area 6; 7A/7P/7PC = area 7A/7P/7PC; 17 = area 17; 18 = area 18; AmCM = subdivision CM of the amygdala; Cbl-V/VIv/IX/IXv = cerebellar lobules V/VIv/IX/IXv; Cd = caudate nucleus; Cu = cuneus; FG = fusiform gyrus; Gpe = external segment of globus pallidus; hIP1-3 = areas hIP1-3; HippFD/SUB = hippocampal subdivisions FD/SUB; hOC4v/hOC5v = ventral parts of areas hOC4/hOC5; IL = insula lobe; IOG/MOG/SOG = inferior/middle/superior occipital gyrus; LG = lingual gyrus; MCC/PCC = middle/posterior cingulate cortex; mFG = medial frontal gyrus; MFG = middle frontal gyrus; MTG = middle temporal gyrus; OP1-4 = operculum; PF/PFm/PFop/PFt/PGa/PGp = areas PF/PFm/PFop/PFt/PGa/PGp in the inferior parietal cortex; Put = putamen; SFG = superior frontal gyrus; TE1.1–3.0 = areas TE1.1–3.0; THp = parietal part of the thalamus. All connectivity matrices are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873; the codes used to transform the fMRI data to networks can be found at http://research.mssm.edu/simonyanlab/analytical-tools/.
Fig 4. Network formation around shared hubs…
Fig 4. Network formation around shared hubs and high-strength nodes of the group-averaged networks at the minimal density of 60% in RSN versus SPN (A) and SPN versus SylPN (B).
Overlap and difference in recruitment of local network communities during rest and sentence production. Red–shared hubs and their connections between the networks; purple–high-strength nodes and their connections in SPN; green–high-strength nodes and their connections in RSN; yellow–high-strength hubs and their connections in SylPN. Abbreviations: L/R = left/right; 3b = areas 3a/3b; 44 = area 44; 4a/4p = anterior/posterior part of area 4; 5Ci = area 5Ci; 5M = area 5M; 6 = area 6; 7A/7P/7PC = area 7A/7P/7PC; 17 = area 17; 18 = area 18; Cbl-VI = cerebellar lobule VI; Cu = cuneus; hIP1-3 = areas hIP1-3; Id1/Ig1 = insular areas Id1 and Ig1; LG = lingual gyrus; MCC/PCC = middle/posterior cingulate cortex; MFG = middle frontal gyrus; MTG = middle temporal gyrus; OP1-4 = operculum; PF/PFm/PFop/PFt/PGa/PGp = areas PF/PFm/PFop/PFt/PGa/PGp in the inferior parietal lobule; Put = putamen; SFG = superior frontal gyrus; TE1.1–3.0 = areas TE1.1–3.0; Tp/pf/s/t/m/pm/v = parietal/prefrontal/somatosensory/temporal/motor/premotor/visual divisions of the thalamus. All connectivity matrices are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873; the codes used to transform the fMRI data to networks can be found at http://research.mssm.edu/simonyanlab/analytical-tools/.
Fig 5. Shared high-degree and high-strength hubs…
Fig 5. Shared high-degree and high-strength hubs in the group-averaged SPN and SylPN at the minimum density of 60%.
(I) Bar charts show strength values of the top 30% strongest nodes (normalized si ranges 1.0–0.9 and 0.89–0.7 in Fig 4) in both the SPN and SylPN. Blue bars highlight nodes that are strength hubs in both SPN and SylPN. (II) Bar charts of the same format show shared-degree hubs of the SPN and SylPN among the top 30% most interconnected nodes (normalized ki ranges 1.0–0.9 and 0.89–0.7 in Fig 5). The table shows nodal values of strength and degree in parentheses. Bold numbers indicate that a node is a hub with respect to the respective metric. Abbreviations: 3b = area 3b; 4a = area 4a; 5M = area 5M; 6 = area 6; 7A = area 7A; LG = lingual gyrus; PCu = precuneus. The corresponding data are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873.
Fig 6. Modular topology of the group-averaged…
Fig 6. Modular topology of the group-averaged networks during the resting state, syllable production, sentence production, sequential finger tapping, and auditory discrimination of pure tones.
Spatially distributed network communities are shown on 3-D brain renderings in the axial and sagittal views and are color-coded based on nodal module affiliation. Nodal size indicates normalized degree; nodes that were removed by the employed elimination strategy are shown in gray. All connectivity matrices are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873; the codes used to transform the fMRI data to networks can be found at http://research.mssm.edu/simonyanlab/analytical-tools/.
Fig 7. Functional community structure of the…
Fig 7. Functional community structure of the group-averaged networks during the resting state, syllable production, sentence production, sequential finger tapping, and auditory discrimination of pure tones.
Network modules are shown as circular groups of nodes positioned around the respective connector hubs, which are arranged on horizontal lines. Only edges passing through connector hubs are drawn, with the respective edge colors indicating the community affiliation of the target nodes. Provincial hubs are displayed as larger circles within their respective native communities. Nodal colors illustrate module membership, where colors are matched to Fig 6. Node lists on the left and right of each graph specify connector and provincial hubs, respectively. Abbreviations: 1 = area 1; 17 = area 17; 2 = area 2; 3a/3b = areas 3a/3b; 44 = area 44; 4a/4p = anterior/posterior part of area 4; 5L/5M = area 5L/5M; 6 = area 6; 7A/7P/7PC = area 7A/7P/7PC; Cbl-V/VI/VIv/VIIa/Cr1 = cerebellar lobules V/VI/VIv/VIIa/Cr1; Cu = cuneus; FG = fusiform gyrus; hIP3 = areas hIP3; IL = insula; SOG = superior occipital gyrus; ITG/MTG = inferior/middle temporal gyrus; LG = lingual gyrus; MCC = middle cingulate cortex; OP1-4 = operculum; PCu = precuneus; PF/PFm/PFop/PFt/PGa/PGp = areas PF/PFm/PFop/PFt/PGa/PGp in the inferior parietal cortex; MFG = middle frontal gyrus; THp/THpf/THpm/THt = parietal/prefrontal/premotor/temporal part of the thalamus; TP = temporal pole; R–right; L–left. All connectivity matrices are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873; the codes used to transform the fMRI data to networks can be found at http://research.mssm.edu/simonyanlab/analytical-tools/.
Fig 8. Schematic overview of (A) whole-brain…
Fig 8. Schematic overview of (A) whole-brain parcellations and (B) fMRI data processing pipeline.
(A) Based on the cytoarchitectonic maximum probability map and macrolabel atlas, the whole brain was parcellated into 212 regions of interest (ROIs), including 142 cortical, 36 subcortical, and 34 cerebellar regions. (B) Voxel-averaged mean time series were extracted from each ROI, and Pearson’s correlation coefficients were calculated for each pair of regions giving rise to 20 correlation matrices for speech production (one for each subject). Six subjects showed inconsistencies during the resting state scan and were removed from further analysis. In a nodal elimination strategy applied to the remaining 14 SPNs, 62 regions were removed from the initial brain parcellation, leaving 150 brain regions for further analysis. The matrices were recomputed and thresholded to obtain a common density range of 77%–86% (10 values, 1% increments). Over this range, group-averaged networks were computed with a density range of 60%–78% (10 values, 2% increments). The sagittal and axial brain views illustrate the relative locations of removed nodes (blue spheres) versus retained nodes (light brown). The same analysis pipeline was applied to the RSN and SylPN. All connectivity matrices are publicly available at http://figshare.com/articles/The_Functional_Connectome_of_Speech_Control/1431873; the codes used to transform the fMRI data to networks can be found at http://research.mssm.edu/simonyanlab/analytical-tools/.

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