[Eeglablist] p300 convention

[Javier Lopez-Calderon]

Here is when old devices for generating calibration signals are so missed!
Best,

Javier

On Jun 17, 2011, at 6:48 AM, Frank H Duffy wrote:

> The clinical EEG tradition of "negative up" (amplifier grid/input one negative, pen goes up) stems from two factors: 
> 
> (1) Most epileptic spikes are scalp negative and
> (2) in a bipolar EEG montage to have a "phase reversal" where the usual negative spike causes adjacent channels in a bipolar montage chain point at each other, negative signals on amplifier input one cause pens to go up.
> You can see how this works in any basic EEG text - see "phase reversal".  
> 
> SO when EPs came along some researchers couldn't wait to re-establish the engineering convention of positive up and some - often using EEG equipment - kept negative up as their convention.
>  
> This polarity convention problem has caused lots of issues. For example some manufacturers, when they provided analogue out voltages for tape recording of EEG signals, inverted the signal and others did not.
> 
> Even today when one well known EEG manufacturer saves data  in EDF format, the signal is voltage inverted while that is not the case for others.  Never trust EDF files from EEG manufacturers if polarity is important to you.  Check because inverted EEG looks alot like non-inverted EEG - eye blinks being the exception.  Some manufacturers don't invert the raw data but set an inverted flag in EDF.  This can be a mess if one does not check polarity convention before starting analysis.
> 
> Best
> 
> Frank
> 
> Frank H. Duffy,MD
> Developmental Neurophysioology
> Childrens Hospital, Boston
> fhd at sover.net
> 
> 
> 
> On 6/16/2011 8:29 AM, mehmet ali wrote:
>> 
>> Hi all,
>> 
>> p300 is conventionally plotted on an inverted axis( positive potentials to down), is there a reason to end up with this convention?
>> 
>> Best,
>> 
>> 
>> 
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Javier Lopez-Calderon
Graduate Student Researcher (GSR)
Center for Mind and Brain           
University of California, Davis
+1 530 574-8330          
javlopez at ucdavis.edu



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