Cortical source estimation of ssVEF responses
Before the ssVEF responses were extracted from the averaged MEG data
epochs, these evoked MEG responses were transformed into cortical source
space by a weighted minimum norm estimation (wMNE) approach (Gramfort et
al. 2014) using the brainstorm toolbox
(https://neuroimage.usc.edu/brainstorm/).
The source reconstruction of evoked MEG data epochs was based on a
forward model based on an overlapping spheres head model (Huang et al.
1999) using a canonical cortical mesh (3003 vertices) derived from the
ICBM152 template (Mazziotta et al. 2001). Thereby, the individual head
and sensor positions of each subject were co-registered with the
template brain by realigning the individual with the template brain’s
fiducials and minimizing the mean distance between the individual head
shape points and the template brain scalp surface (Moratti et al. 2011).
Thereafter, the forward model was calculated by using a head model based
on overlapping spheres (Huang et al. 1999). Finally, a weighted Minimum
Norm Estimation (wMNE) (Gramfort et al. 2014) was used to calculate the
current density of the evoked MEG averages (see above) on the cortical
surface for later analysis. Finally, for illustration purposes and later
statistical analysis, regions of interests (ROIs) were defined by
overlaying a co-registered Brodmann Atlas as implemented in Brainstorm
(Tadel et al., 2011;https://neuroimage.usc.edu/brainstorm/).
The atlas contains V1, V2, and hMT+ areas and were extended by adding
inferior-temporal visual cortex and the occipito-parietal brain area
(see Figure 1 below).
In order to extract the ssVEF responses in cortical source space, the
time series of the cortical current densities at each vertex of the
cortical surface were demeaned and the 12 Hz and 15 Hz Fourier
components were determined by a Fast Fourier Transform (FFT) using the
fieldtrip toolbox (Oostenveld et al. 2011);https://www.fieldtriptoolbox.org).
For the pre-cue baseline conditions (attend the central fixation cross
vs. attend both peripheral rings), the 1 s epochs were zero padded to 10
s in order to obtain a frequency resolution of 0.1 Hz for the FFT. For
the post-cue 2.9 s epochs overlapping four 1 s intervals were extracted
(0.5 s to 1. 5 s, 1 s to 2 s, 1.5 s to 2.5 s and 2 s to 2.9 s). Each
interval was zero padded to 10 s in order to obtain a 0.1 Hz frequency
resolution and submitted to FFT analysis. Then, the obtained FFTs were
averaged. Finally, for all FFTs (pre-cue baseline and post-cue
intervals) the 12 Hz and 15 Hz power of the corresponding Fourier
components were determined at each vertex of the canonical brain surface
mesh.