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

[Makoto Miyakoshi]

Dear Masaki,

Your comment is welcome!

Makoto

On Tue, Oct 28, 2014 at 10:04 PM, Dr.Rohit Tyagi <
rohittyagi_2k at rediffmail.com> wrote:

> 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,
> Rohit
>
>
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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