Hello Ahmed,

Does the same thing happen while you record VEP of other patients?

If yes, I assume there must be something wrong with your settings. Did you use pattern reversal stimulus or pattern onset/offset stimulus? The waveforms of pattern reversal and pattern onset/offset stimulus are opposite. Please see the attached reference: http://www.iscev.org/standards/pdfs/ISCEV-VEP-Standard-2010.pdf

I hope that helps.

Regards,

Nabin

Did you include a figure in your original communication? This will help.

The occipital scalp lead needs to be plugged into positive, and reference into negative to have "P100" up.

The polarity of the VEP to a checkerboard depends heavily on the spatial frequency of the pattern: with a high SF you will see a strong negative polarity across Oz and other central scalp sites. See attached paper for an example.

Article Selective attention to spatial frequency: An ERP and source ...

Dear Donnell Joseph Creel,

Thanks for your comment, I'm wonder how can we reverse the leads polarity if we used common reference for all electrodes (we used dense electrodes cap)?

Dear Mr. Johanna, thanks for such a nice paper.  Its sound the peaks at 100 ms of all check sizes with a negative polarity. Peak amplitude vary with SF but not the polarity.