Determination of cortical sources that significantly oscillated at the stimulus driven frequencies (12 Hz and 15 Hz)
We determined which cortical sources showed oscillatory neural activity at the driving ssVEF frequencies significantly different from background noise. As power estimates are the sum of two squared measures (real and imaginary parts), power estimates of the signal (here at the ssVEF frequency of interest) and noise (here at frequency bins different from the ssVEF frequencies of interest) follow a chi-square distribution (Dobie and Wilson 1996). Therefore, the power at the frequency of interest can be tested against noise by an F-ratio test with degrees of freedom df = 2 for the numerator and df = 2m for the denominator, whereby m is the number of noise frequency bins (Dobie and Wilson 1996).
Using this method, we created two masks for the pre-cue central fixation cross task at 12 Hz and 15 Hz (flicker frequency for each visual hemifield was counter-balanced across subjects) based on the grand average power spectrum across all subjects, representing our signal. Noise was defined by 31 noise frequency bins between 17 Hz and 20 Hz (0.1 Hz steps), that were chosen as they did not contain any harmonics of the ssVEF responses. The sources included in the masks were parametrically determined by an F test with the corresponding degrees of freedom at a false discovery rate corrected alpha threshold of 0.05 (3003 comparisons, threshold at F(2, 62) = 9.03, pfdr corrected < 0.001). Then, these two masks were OR logically combined into one mask for the pre-cue central fixation cross task. The same was done for the pre-cue peripheral rings task (threshold at F(2, 62) = 10.12, pfdr corrected < 0.001). The two masks were created in order to compare the spatial extension of significant ssVEF responses in cortical source space between these two pre-cue baseline tasks (see Results).
Finally, a third mask was created by logically OR combining the source masks of the F-ratio tests on the grand means of power spectra of the fully balanced combinations of pre- and post-cue conditions (e. g. pre-cue baseline attend rings, post-cue lvf not attended, post-cue lvf attended, pre-cue baseline attend cross, post-cue lvf not attended, post-cue lvf attended, etc.). This was done in order not to bias the implicated sources in the mass-univariate statistical tests (see below) towards one pre- and post-cue task combination. The same parameters as described above were applied to these F-ratio tests (threshold at F(2, 62) = 8.69, pfdr corrected < 0.001). Figure 1 depicts the resulting overall mask for the overall statistical comparisons (see below).