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Hierarchical Frequency Tagging animation

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modified on 31.10.2016, 01:32
An 8-second animated movie representation of the Hierarchical Frequency Tagging method using 1.3Hz for the SWIFT cycles and 10Hz for the SSVEP (the second movie provides a slow-motion demonstration of the first few seconds).

The trial was constructed using one house and one face SWIFT sequence. In order to preserve low level image attributes within a given trial, regardless of the specific images presented in each cycle, we created and merged additional ‘noise’ sequences in the following way. First, we randomly selected one of the scrambled frames from each of the original SWIFT sequences. Then, we created noise sequences by applying the SWIFT method on each of the selected scrambled frames. In this way, each original ‘image’ sequence had a matching ‘noise’ sequence. Finally, each ‘image’ sequence was alpha blended with the ‘noise’ sequences of the other image with equal weights in luminance value at each pixel at any given frame. This way, for example, cycles in which a face image was to appear contained the face image superimposed with a house noise sequence. The overall low level visual attributes were therefore constant across all frames in the trial regardless of the identifiable image in each cycle, ensuring that the frequency tagged signal would not reflect local physical input properties.

This video shows an 8-second section of the resulting ‘movie’ in which images peaked in a cyclic manner (1.3Hz) in a pseudorandom order. A global sinusoidal contrast modulation at 10Hz was applied on the whole movie sequence to evoke the SSVEP.