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The clinical EEG tradition of "negative up" (amplifier grid/input
one negative, pen goes up) stems from two factors: <br>
<br>
(1) Most epileptic spikes are scalp negative and<br>
(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.<br>
You can see how this works in any basic EEG text - see "phase
reversal". <br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Best<br>
<br>
Frank<br>
<br>
Frank H. Duffy,MD<br>
Developmental Neurophysioology<br>
Childrens Hospital, Boston<br>
<a class="moz-txt-link-abbreviated" href="mailto:fhd@sover.net">fhd@sover.net</a><br>
<br>
<br>
<br>
On 6/16/2011 8:29 AM, mehmet ali wrote:
<blockquote
cite="mid:BANLkTimyKO5W55Ad-88w9ixPq2Www_uBJQ@mail.gmail.com"
type="cite">Hi all,<br>
<br>
p300 is conventionally plotted on an inverted axis( positive
potentials to down), is there a reason to end up with this
convention?<br>
<br>
Best,<br>
<br>
<br>
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