Spatial Resolution and Imaging Encoding fMRI Settings for Optimal Cortical and Subcortical Motor Somatotopy in the Human Brain.

Renaud Marquis,Sara Lorio,Bogdan Draganski,Lester Melie-Garcia,Ferath Kherif,Borja Rodriguez-Herreros,Sandrine Muller,Antoine Lutti

doi:10.3389/fnins.2019.00571

Abstract

There is much controversy about the optimal trade-off between blood-oxygen-level-dependent (BOLD) sensitivity and spatial precision in experiments on brain’s topology properties using functional magnetic resonance imaging (fMRI). The sparse empirical evidence and regional specificity of these interactions pose a practical burden for the choice of imaging protocol parameters. Here, we test in a motor somatotopy experiment the impact of fMRI spatial resolution on differentiation between body part representations in cortex and subcortical structures. Motor somatotopy patterns were obtained in a block-design paradigm and visually cued movements of face, upper and lower limbs at 1.5, 2, and 3 mm spatial resolution. The degree of segregation of the body parts’ spatial representations was estimated using a pattern component model. In cortical areas, we observed the same level of segregation between somatotopy maps across all three resolutions. In subcortical areas the degree of effective similarity between spatial representations was significantly impacted by the image resolution. The 1.5 mm 3D EPI and 3 mm 2D EPI protocols led to higher segregation between motor representations compared to the 2 mm 3D EPI protocol. This finding could not be attributed to differential BOLD sensitivity or delineation of functional areas alone and suggests a crucial role of the image encoding scheme – i.e., 2D vs. 3D EPI. Our study contributes to the field by providing empirical evidence about the impact of acquisition protocols for the delineation of somatotopic areas in cortical and sub-cortical brain regions.

Highlights

We report activations in primary motor cortical areas, thalamus, putamen and pallidum
We demonstrate the impact of functional magnetic resonance imaging (fMRI) protocol settings on neural activity patterns in cortical and subcortical brain regions
Given that previous studies demonstrated the impact of fMRI data pre-processing (Geissler et al, 2005), experimental design(Besle et al, 2013b) and statistical analysis (Dechent and Frahm, 2003) on topology properties of neural activity, we kept these parameters unchanged across spatial resolutions

Summary

Introduction

Whilst electrophysiological studies provide strong evidence that somatotopy representations in the basal ganglia and thalamus are spatially segregated (Alexander and DeLong, 1985; Nambu, 2011), fMRI studies failed to robustly replicate these findings (Lehéricy et al, 1998; Maillard et al, 2000; Gerardin et al, 2003; Staempfli et al, 2008; Oguri et al, 2013; Zeharia et al, 2015). The increase in spatial resolution leads to drop in sensitivity to the BOLD effect that can be partially compensated only for cortical regions using multi-channel receive coils (Triantafyllou et al, 2005, 2011) This is supported by somatotopy studies consistently showing high level of segregation in primary motor cortex (Kapreli et al, 2007; Meier et al, 2008; Zeharia et al, 2012; Cunningham et al, 2013) and SPM (Indovina and Sanes, 2001; Strother et al, 2012), but failing to obtain similar results in deep brain nuclei. In group-level analysis, this marked reduction augments the inter-individual variability that results in poor differentiation between somatotopy areas (Scholz et al, 2000)

Methods

Results

Conclusion