[Eeglablist] Navigating ERP waves

Stephen Politzer-Ahles politzerahless at gmail.com
Wed Aug 28 00:27:13 PDT 2019


Hi Emmanuelle,

First of all, you should keep in mind that peaks you see in the wave do not
necessarily correspond to components. So the fact that you see a
negative-going local peak around 100 ms doesn't necessarily mean you are
seeing an N100, etc. See Luck (e.g.,
https://www.researchgate.net/publication/220049326_Ten_simple_rules_for_designing_ERP_Experiments).
Typically, if your hypothesis is about a difference between ERPs elicited
under different stimulus conditions, then it only makes sense to try to
interpret the differences between the waves, not the peaks within the
individual waves.

Secondly, polarity reversals are normal because EEG activity comes from
dipoles, so typically if there is positive on one side of the head there
must be negative on the opposite side. (In the case of effects that seem to
be the same polarity over most of the head, like the classic N400 effect
[negative over pretty much the whole head, if the reference was the average
of the mastoids] then actually there's also a positivity underneath the
head, but we generally don't have electrodes there since the neck is
there.) This also depends on your reference, which you haven't mentioned;
changing the reference can greatly change what your ERPs look like. See
e.g. Dien (
https://www.sciencedirect.com/science/article/abs/pii/S0167876016306985).

Now, there are also some artifacts which may show a strong reversal between
frontal and posterior channels, e.g. I think I've seen that before with
heartbeats. But without seeing a snapshot of your data (like topoplots at
several time points) it's not easy to tell if there are problems or not
based on your description of what you see in your data.



---
Stephen Politzer-Ahles
The Hong Kong Polytechnic University
Department of Chinese and Bilingual Studies
http://www.mypolyuweb.hk/~sjpolit/
<http://www.nyu.edu/projects/politzer-ahles/>


On Wed, Aug 28, 2019 at 3:06 AM Emmanuelle Renauld <
emmanuelle.renauld.1 at ulaval.ca> wrote:

> Hi everyone,
>
>
> I am trying to understand if my results are good for my ERP study. My
> litterature goes all over the place so I have some difficulties when it
> comes to naming what I see.
>
> I find that I have generally exactly the same pattern (with very similar
> latency) in frontal vs posterior electrodes, only with reverse polarity.
> Frontal:          ~N100          ~P200            ~N300
> Posterior:      ~P100           ~N200           ~P300
>
> Is that something often seen? I have a visual task with a condition of
> craving. I wondered briefly if it could be due to my average re-referencing
> (64 electrodes montage) but from what I read, it shouldn't be the case.
>
> Thanks!!
>
>
> Emmanuelle
>
>
>
>
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