[Eeglablist] p300 convention

[Frank H Duffy]

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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