Elsevier

NeuroImage

Volume 145, Part A, 15 January 2017, Pages 107-117
NeuroImage

High-resolution retinotopic maps estimated with magnetoencephalography

https://doi.org/10.1016/j.neuroimage.2016.10.017Get rights and content

Highlights

  • A method for estimating visual receptive fields.

  • Estimation of retinotopic organization of the primary visual cortex using MEG.

  • Given optimal stimulation and brain curvature, MEG can achieve spatial resolution comparable to fMRI.

Abstract

Magnetoencephalography (MEG) is used in clinical and fundamental studies of brain functions, primarily for the excellent temporal resolution it provides. The spatial resolution is often assumed to be poor, because of the ill-posed nature of MEG source modeling. However, the question of spatial resolution in MEG has seldom been studied in quantitative detail. Here we use the well-known retinotopic organization of the primary visual cortex (V1) as a benchmark for estimating the spatial resolution of MEG source imaging. Using a standard visual stimulation paradigm in human subjects, we find that individual MEG sources exhibit well-delineated visual receptive fields that collectively follow the known mapping of the retinal surface onto the cortex. Based on the size of these receptive fields and the variability of the signal, we are able to resolve MEG signals separated by approximately 7 mm in smooth regions of cortex and less than 1 mm for signals near curved gyri. The maximum resolution is thus comparable to that of the spacing of hypercolumns in human visual cortex. Overall, our results suggest that the spatial resolution of MEG can approach or in some cases exceed that of fMRI.

Introduction

Among the various methods for non-invasive imaging, magnetoencephalography (MEG) source imaging is known to provide outstanding temporal resolution, while it is typically assumed to have modest spatial resolution (Darvas et al., 2004, Hämäläinen et al., 1993). Consequently MEG imaging is most often used in experiments aimed at measuring temporal fluctuations in neural signals for which the assignment of a precise anatomical source is not critical. Although recent reports have raised the possibility of extracting rich spatial signals from MEG (Cichy et al., 2015), a quantitative estimate of the resolution that can be attained with this imaging modality is lacking.

Here we have examined the capacity of MEG to resolve the well-known retinotopic organization of the primary visual cortex (V1). This representation provides a useful benchmark, because it has been thoroughly and quantitatively characterized using a variety of methods, including electrophysiology (Das and Gilbert, 1995, Hubel and Wiesel, 1977), PET (Fox et al., 1987), optical imaging (White and Culver, 2010) and fMRI (Engel et al., 1997). These approaches have demonstrated a smooth change in the locus of cortical activation for corresponding changes in the position of the retinal stimulus (Dumoulin and Wandell, 2008, Engel et al., 1997, Sereno et al., 1995). Thus to the extent that an imaging modality has high spatial resolution, it should be able to differentiate responses to visual stimuli in different locations. The smallest shift in the locus of cortical activation that can be detected serves as a measure of resolution.

Here we have obtained retinotopic maps from human subjects using MEG in combination with a standard visual stimulation paradigm. We show that surprisingly high spatial resolution maps can be obtained with appropriate choices of visual stimulation and source modeling. In particular, we are able to reliably detect distinct MEG responses emanating from sources separated by 7.0 mm along smooth cortical surfaces and less than 1 mm along the arched gyri.

Section snippets

Participants

Data were recorded from two healthy, right-handed male subjects (one author, one naïve), both of whom had normal or corrected to normal vision. Both subjects gave written consent prior to participation in three sessions, involving structural MRI, functional MRI, and MEG recordings. All experimental protocols were approved by the Research Ethics Board of the Montreal Neurological Institute.

Structural MRI

For the MRI scans, each subject was positioned on his back with a 32 channel surface coil centered over the

Results

We studied the spatial properties of MEG signals emanating from the visual cortex in two human subjects. We elicited visual responses by presenting images comprised of a small number of squares flashed simultaneously on a computer monitor. In this section we analyze the relationship between the positions of individual stimulus squares and MEG source responses, as well as the distribution of these receptive fields across the cortical surface; we use these data to derive an estimate of the

Brief summary of results

In this study we have demonstrated the capacity of individual MEG sources to show selectivity for specific areas of the visual field. We showed that localized visual receptive fields for individual sources (Fig. 3) can be obtained from modest amounts of data (Appendix 3), and that the ensembles of these receptive fields form orderly maps within the occipital lobe (Fig. 4, Fig. 5). These maps are well matched to those obtained with fMRI (Fig. 7). Analysis of correlated responses between pairs of

Conclusion

MEG has traditionally been used in applications requiring excellent temporal resolution. However, its spatial resolution is most often considered to be coarse. We have shown that MEG can recover retinotopic maps with similar shape to those obtained with fMRI, and in some areas with comparable spatial resolution.

Acknowledgements

We would like to thank Elizabeth Bock for MEG training and help in data acquisition, François Tadel for valuable suggestions in using Brainstorm and Drs. Matthew Krause and Theodore Zanos for their comments on the data analysis and Dr. Reza Farivar-Mohseni for his help with fMRI surface rendering. This work was funded by a Molson Neuro-Engineering and a Gerry Sklavounos - MNA Laurier-Dorion Scholarship to K.N., and by a grant from NSERC (341534-12) to C.C.P. S.B. was supported by the Killam

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