Strange scalp distribution maps for P1

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icke
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Joined: Mon Sep 27, 2010 12:25 pm
Location: Haifa

Strange scalp distribution maps for P1

Post by icke »

I am not sure whether the phenomenon described below is a technical artifact or an occurrence within normal range. The Experiment: visual lexical decision task. Recording with ActiveTwo 64 electrodes. Stimulus presentation: Presentation. ERP processing with Vision Analyzer in a nutshell: 1. merge bit-markers 2. filter (0.1 – 25 Hz) 3. average reference 4. ocular correction with Graton and Cole method 5. stimulus locked segmentation for words and pseudo-words separately 6. artifact rejection 7. averaging 8. baseline correction (-100 – 0 prior stimulus onset)

The ERP wave forms resulting from this experiment/processing (for a relatively big sample of N = 50) look clean and nice. However, t[b]he scalp maps for the P1 component look strange. That is, in addition to strong bilateral positive activity in parietal/occipital areas (PO7 and PO8) there is strong negative activity within in central occipital areas (Oz).[/b] Btw, Current Source Density Maps and 2D Maps look almost identical in the P1 time segment. Now this looks very different from scalp maps we have seen for comparable experiments using different equipment in the past. So here my questions:
1. Do other Biosemi users also observe these visual P1 scalp maps with the occipital/central negative activity?
2. Could it be that the placement of the DRL/CMS around POz plays in a role here?
3. Can anyone spot an obvious error in our processing routine?
Thanks in advance for any help or suggestion,

Sebastian

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