[Eeglablist] Navigating ERP waves

[Makoto Miyakoshi]

Dear Emmanuelle and Stephen,

> My litterature goes all over the place

Hmm that is a genius expression, very impressive.

I agree with Stephen, it's necessary to show the waveform in question for
meaningful discussion.

Makoto

On Wed, Aug 28, 2019 at 12:40 AM Stephen Politzer-Ahles <
politzerahless at gmail.com> wrote:

> 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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-- 
Makoto Miyakoshi
Assistant Project Scientist, Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego