[Eeglablist] Stimulus design for SSVEP with flickering or reversing checkerboards

[David Bridwell]

The frequency of physical stimulation is 5 Hz (i.e. the fundamental
frequency). That is the full cycle of the square wave within any individual
pixel on the display. There will also be EEG responses at twice the
fundamental frequency (i.e. 10 Hz) as you point out. A Fourier transform of
the square wave input will display peaks at additional frequencies (i.e. 15
Hz, 20 Hz, and so on, although they will attenuate with higher
frequencies), thus it is expected that the brain might generate responses
within those frequencies as well, although they will be pretty small. I
would expect that your display will generate the largest response at 10 Hz.

In some initial piloting I found larger responses to twice the fundamental
frequency (i.e. F2) of a contrast flicker compared to the fundamental
frequency of a luminance flicker. In addition, the F2 response to a
contrast flicker appears to show the greatest sensitivity to attention, in
particular when it fell between 8 and 12 Hz. Thus, I focused on these
responses in my attention studies:

Bridwell, D.A., Srinivasan, R. (2012) *Distinct attention networks for
feature enhancement and suppression in vision. * Psychological Science,
23(10):1151-1158.'

Bridwell, D.A., Hecker, E.A., Serences, J.T., Srinivasan, R. (2013) *Individual
differences in attention strategies during detection, fine discrimination,
and coarse discrimination.* Journal of Neurophysiology, 110:784-794

Here is a paper that demonstrates/discusses this is better detail:

Kim, Y.J., Grabowecky, M., Paller, K.A., Suzuki, S. (2011) *Differential
roles of frequency-following and frequency doubling visual responses
revealed by evoked neural harmonics.* Journal of Cognitive Neuroscience,
23:1875-1886.
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