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Hierarchical Frequency Tagging animation
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modified on 2016-10-31, 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.