[Eeglablist] Discrepancy between STUDY results and manual averaging

[Arnaud Delorme]

Dear James,

the menu "Plot-->Sum/compare ERPs" inverts polarity by default while the STUDY functions do not. Could this be the issue? I have tried to plot electrode FCz of the tutorial STUDY data using the "Plot-->Sum/compare ERPs" and using the STUDY menu and was not able to reproduce the issue (see screen capture, the top being "Plot-->Sum/compare ERPs" and the bottom one being the STUDY). This was with EEGLAB version 12. Would you mind to try with EEGLAB 11 and 12 to make sure the issue persist (please email me directly so as not to clutter the mailing list)? Then the next step is to upload your datasets to our Bugzilla database so we can have a closer look.

Thanks,

Arno




On 19 Oct 2012, at 13:49, James Jones-Rounds wrote:

> Hello all,
> 
> Has anyone else noticed a discrepancy between ERPs generated with the STUDY function (using EEGLAB v.9.x) and ERPs generated by loading all your datasets and using "Plot-->Sum/compare ERPs" ?
> 
> Using Plot>Sum/compare ERPs we found an expected P300, increasing in amplitude as we moved from frontal to parietal midline electrodes. Using a STUDY design, however, we found at these same electrodes a strange pattern of inverted polarity at adjacent electrodes (i.e. clear P300 at FPZ, inverted P300, aka an "N300", at FZ,) back and forth down the midline.
> 
> Some notable points: a) we generated a STUDY design using participant data files that were separated by condition (i.e. subj1_cond1, subj1_cond2, subj2_cond1, subj2_cond2, etc.); b) the data were referenced to the average of all electrodes (except the mastoid, which served as the original reference). They were collected on a 64 channel NeuroScan unit with the 10-20 electrode positioning.
> 
> Thank you for any comments and suggestions you might have!
> 
> James
> 
> -- 
> Laboratory Manager
> Human Development EEG & Psychophysiology (HEP) Laboratory,
> Department of Human Development,
> --------------------------------------------
> Technician and Laboratory Safety Officer
> Computational Physiology Laboratory,
> Department of Neurobiology & Behavior,
> --------------------------------------------
> Cornell University | Ithaca, NY
> 
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