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. 2019 Apr 30;17(4):e3000233. doi: 10.1371/journal.pbio.3000233

Fig 2. Stimulus construction and analysis methodologies.

Fig 2

(A) All experiments implemented the HFT method using face and house images. SWIFT sequences are presented at given frequencies, allowing tagging of image-recognition activity (red rectangles). Contrast modulation is applied at a higher frequency, inducing SSVEP (blue sinusoid). When analysing the EEG data in the frequency domain (bottom graph with multiple peaks), peaks in the power spectrum can be seen at the fundamental frequencies and their harmonics (red bars for SWIFT f1 and blue bars for SSVEP f2). Additional peaks at IM components (e.g., purple bars for f2 + f1 and f2 − f1) are suggested to indicate integration of bottom-up SSVEP signals with top-down SWIFT signals. (B) The MSPC [36] quantifies the degree to which an IM frequency component is driven by the phases of the fundamental input frequencies. In other words, the degree to which the IM component reflects an interaction between those input frequencies. Within each epoch, we first calculate the difference between the sum of the (weighted) phases of the fundamental input frequencies and the phase of the IM component. Then, we compute the coherence of this value across multiple epochs applying the same method as in the well-known phase-locking value (see Methods for a detailed description.) Here, we introduced a novel distinction between two measures—MSPCstim and MSPCres—which differ in what we consider the ‘input’ signals to be. Specifically, the MSPCstim ties the IM phase to the phases of the stimulus itself (i.e., the images presented on the screen), while MSPCres ties it to the phases of the tagged neural responses (See Methods). We suggest that these measures distinguish between neural interactions occurring at lower and higher cortical levels, respectively. EEG, electroencephalography; HFT, hierarchical frequency tagging; IM, intermodulation; MSPCres, multispectral phase coherency (response); MSPCstim, MSPC (stimulus); SSVEP, steady-state visual evoked potentials; SWIFT, semantic wavelet-induced frequency-tagging.