Retinotopic mapping of the primary visual cortex - a challenge for MEG imaging of the human cortex

Abstract

Magnetoencephalography (MEG) can be used to reconstruct neuronal activity with high spatial and temporal resolution. However, this reconstruction problem is ill-posed, and requires the use of prior constraints in order to produce a unique solution. At present there are a multitude of inversion algorithms, each employing different assumptions, but one major problem when comparing the accuracy of these different approaches is that often the true underlying electrical state of the brain is unknown. In this study, we explore one paradigm, retinotopic mapping in the primary visual cortex (V1), for which the ground truth is known to a reasonable degree of accuracy, enabling the comparison of MEG source reconstructions with the true electrical state of the brain. Specifically, we attempted to localize, using a beanforming method, the induced responses in the visual cortex generated by a high contrast, retinotopically varying stimulus. Although well described in primate studies, it has been an open question whether the induced gamma power in humans due to high contrast gratings derives from V1 rather than the prestriate cortex (V2). We show that the beanformer source estimate in the gamma and theta bands does vary in a manner consistent with the known retinotopy of V1. However, these peak locations, although retinotopically organized, did not accurately localize to the cortical surface. We considered possible causes for this discrepancy and suggest that improved MEG/magnetic resonance imaging co-registration and the use of more accurate source models that take into account the spatial extent and shape of the active cortex may, in future, improve the accuracy of the source reconstructions.

Fig. 1

The experimental stimulus.

Fig. 2

Plot of peak pseudo-T values in the 30–60 Hz frequency range for each angular location for four subjects (AH, SW, OL and SH), as well as the group mean with bars showing the standard error of the mean. Peak voxels with pseudo-T < 0.5 were not included in the analysis. Locations are defined so that 0° and 360° represent the vertical position in the lower half of the visual field. AH and SW were the two subjects selected for further analysis, whereas OL and SH are examples of subjects with a weak response.

Fig. 3

Plots of the Spearman's rank correlation coefficient between the angular location of the stimulus and the angular position of the corresponding source for each participant. Each panel shows the correlation separately for one of the four visual quadrants. ^×Quadrants for which less than three peaks were recovered and so no correlation coefficient could be calculated. *Correlations that were significant at the P < 0.05 level.

Fig. 4

Locations of the strongest cortical source (peak pseudo-T) in the 30–60 Hz frequency range as the stimulus rotated around the visual field, for subject AH (top row) and subject SW (bottom row). The central panel in each row shows a close-up of the sources from a coronal view, whereas the smaller panels show the data against a full cross-section of the cortical gray/white matter boundary from coronal (left) and axial (right) views. Each source location is represented in the figure by a ‘clock face’ centered on the location of the peak pseudo-T value, with the position of the line(s) in each ‘clock’ indicating the angular location(s) within the visual field at which the stimulus produced that peak.

Fig. 5

Flattened surfaces extracted from the medial occipital cortex in the right and left hemispheres of participants AH and SW. Each point on the surface is colored according to preference for the angular location of the stimulus, as derived from volumetric images of pseudo-T magnitude. The white line and asterisk on each surface indicate the approximate locations of the fundus of the calcarine sulcus and the occipital pole, respectively.

Fig. 6

Locations of cortical sources (peak pseudo-Z) in the 30–60 Hz frequency range as the stimulus rotated around the visual field, for subject AH (top row) and subject SW (bottom row). The central panel in each row shows a close-up of the sources from a coronal view, whereas the smaller panels show the data against a full cross-section of the cortical gray/white matter boundary from coronal (left) and axial (right) views. Each source location is represented in the figure by a ‘clock face’ centered on the location of the peak pseudo-Z value, with the position of the line(s) in each ‘clock’ indicating the angular location(s) within the visual field at which the stimulus produced that peak.

Fig. 7

Locations of the strongest cortical source (peak pseudo-T) in the 4–7 Hz frequency range as the stimulus rotated around the visual field, for subject AH (top row) and subject SW (bottom row). The central panel in each row shows a close-up of the sources from a coronal view, whereas the smaller panels show the data against a full cross-section of the cortical gray/white matter boundary from coronal (left) and axial (right) views. Each source location is represented in the figure by a ‘clock face’ centered on the location of the peak pseudo-T value, with the position of the line(s) in each ‘clock’ indicating the angular location(s) within the visual field at which the stimulus produced that peak.

Fig. 8

Locations of the strongest cortical source (peak pseudo-T) in the 30–60 Hz frequency range as the stimulus rotated around the visual field, measured in the second session with subject SW. The central panel shows a close-up of the sources from a coronal view, whereas the smaller panels show the data against a full cross-section of the cortical gray/white matter boundary from coronal (left) and axial (right) views. White ‘clocks’ show results from the trial with stimulus radius of 3.75°, and gray ‘clocks’ show results from the trial with stimulus radius of 1.25°.