[Eeglablist] Statistics for ERSP and ITC

[Makoto Miyakoshi]

Dear Laurent,

Wow these are two counterintuitive plots, I agree with you.
If I were you, I would check the following two things

   1. What's your baseline period in these time-frequency plots? If you use
   the default, all negative latencies are included, but that would contain
   half of your spike period.
   2. How many trials do you have for each condition? Particularly for ITC,
   SNR of 'background' is directly affected by the number of trials (since the
   resultant vector length is normalized to 0-1, data noisiness/quietness in
   the 'background' is a direct function of number of trials. If two
   conditions compared have different number of trials, you cannot make exact
   comparison--sometimes all the 'background' (which I means is time-frequency
   window of non-interest) will light up simply because of different noise
   level. I'm not 100% sure though if this is the case in your example).

I have never used 'use all single trials...' option, so I can't say for
sure about its effect, but assuming that it is irrelevant here.

Generally speaking, ITC is a cranky measure compared with ERSP. You should
be careful.

If it does not make sense, please let me know.

Makoto

On Mon, Jun 29, 2020 at 10:28 AM Laurent Sheybani <Laurent.Sheybani at unige.ch>
wrote:

> Dear Eeglabers,
>
> I am working on an EEG dataset of a cognitive task and I would like to see
> whether there is an increase in power and/or ITC around subjects' answers.
> Before analysing these data, I am getting used to the EEGLAB gui using
> epileptic data that I acquired in the past, just to see whether I process
> my data correctly in eeglab. I marked all interictal epileptic discharges,
> so that I know when there is an increase in power.
>
> What is very strange is that I find no significant increase in power over
> the whole window (-2000 to 1000 around interictal epileptic discharges,
> 3-50 Hz), while the whole window is significant for ITC. I would have
> expected a significant increase in power around interictal epileptic
> discharges (as I see it in individual, raw LFP activity) and if there was
> an increased ITC, I would have expected it to be more specific (in terms of
> time and frequency range).
>
> The pipeline that I follow is:
>
>   *   Load data into EEGLAB from a .mat file (1x28822528): 1 electrode,
> 28822528 timeframes at 16000 Hz sampling frequency.
>   *   Load the event file
>   *   Downsampling the data to new sampling frequency = 100 Hz.
>   *   Export epochs of "spike" and "control" period (-2000 to 1000)
>   *   Load these .set file into a study (1 subject, 1 session, 2
> conditions)
>   *   Study design: Categorical variable: condition - Values (control -
> spike)
>   *   Precompute channel measures, selecting ERPs, power spectrum,
> ERP-image, ERSP and ITC (options for ICA are not chosen, as I have only 1
> EEG trace), I keep all default parameters
>   *   Plot channels measures:
>      *   Stats:
>         *   I check "Compute 1st independent variable statistics if any"
> and "Use single trials for statistics" (I have only 1 subject)
>         *   Use EEGLAB statistics, permutation statistics, FDR correction,
> statistical threshold 0.05
>         *   Params: I do not change the parameters.
>
> As you can see (power file:
> https://urldefense.com/v3/__https://www.dropbox.com/s/rwh3b2u8doc8oab/power.jpg?dl=0__;!!Mih3wA!R-vXecxt4S9nEJwpZ1Y3TT5lFDzVkvCPQW8ctGvZlHkOpF7Adw-hqNe9c3QF6ZzjUvrFqg$
> ) (ITC file:
> https://urldefense.com/v3/__https://www.dropbox.com/s/npa6f7llnv6tbhl/ITC.jpg?dl=0__;!!Mih3wA!R-vXecxt4S9nEJwpZ1Y3TT5lFDzVkvCPQW8ctGvZlHkOpF7Adw-hqNe9c3QF6Zwd2CJ8HA$
> ) there is no increase in power, while there is a non-specific change in
> ITC.
>
> Would any one know where in my processing I did a mistake ? It seems that
> the ERSP/ITC plots themselves are ok, but the p-values plot are not ?
>
> Thanks in advance for your help !
>
> -----------------------------
> Dr. Laurent Sheybani, Ph.D
> Médecin interne en Neurologie Clinique
> Clinique de Neurologie
> Hôpitaux Universitaires de Genève
> Rue Gabrielle-Perret-Gentil, 4
> 1205 Genève
> Switzerland
>
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