Trial time-courses were extracted from bilateral visual area V1, defined using a multi-modal parcelation from the Human Connectome Project, which combined retinotopic mapping, T1/T2 structural MRI and diffusion-weighted MRI to accurately define the boundaries between cortical areas (Glasser et al., 2016; Figure 3C). The downsampled version of this atlas can be found in the parent directory of the Figshare repository (see later). To obtain a single spatial filter from this region, we performed a principle components analysis (PCA) on the concatenated filters from 182 vertices of bilateral V1, multiplied by the sensor-level covariance matrix, and extracted the first component. The sensor-level data was then multiplied by this spatial filter to obtain a V1-specific “virtual electrode” (script: 3_get_VE_frontiers_PAC.m), and the change in oscillatory power between grating and baseline periods was calculated from 1 to 100 Hz, using a 500 ms time window, sliding in steps of 20 ms and ±8 Hz frequency smoothing (script: 4_calc_pow_change.m). It is important to note that while we decided to use a multimodal atlas, visual area V1 virtual electrode time-series could also be defined using a more standard volumetric approach, for example the AAL atlas, which is included in the Fieldtrip toolbox (Oostenveld et al., 2010).
