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

Dr.Rohit Tyagi rohittyagi_2k at rediffmail.com
Tue Oct 28 22:04:58 PDT 2014

Hi Everyone,

I have a very simple question, because of a confusion raised by a colleague. We are studying the frequency spectral density characteristics of SSVEP EEG data. The stimuli that we use are a pair of complimentary black-white checkerboards, which are shown alternately at a frequency of 10 Hz to induce SSVEP on an LCD screen.

One checkerboard (pic1) is shown for 100ms, followed by its inverse (pic2) for 100ms and so on for the whole trial duration of about 30-40 seconds.

Here is where a difference of opinion has cropped up - 

My colleague says that since I am presenting pic1 only 5 times in this one second, so the frequency of stimulation is not 10Hz but 5 Hz. I believe that what matters in such a paradigm is the frequency of reversal which will drive the brain, so the effective frequency is 10 Hz.

Using spectopo function, I can plot the frequency characteristics of the EEG data and it can show a clear peak at 10 Hz using this stimulation, however I wanted to confirm whether my understanding of generating SSVEP with reversing checkerboard is correct.

On a second issue, say I was using just one checkerboard (pic1) and did an onset/offset stimulation - where the checkerboard appears on the LCD (with gray background) for say 50ms, then disappears for 50ms and so on - in this case I can present the same picture for 10 times in a second, but I suspect that the offset also counts as a "stimulation" event and thus the spectral response might show a peak at 20 Hz.

Any clarifications would be very helpful.

Thanks and Regards,


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