[Eeglablist] When to select Additive Baseline in ERSP Study Function?

[Makoto Miyakoshi]

Dear Kathleen,

> Can someone explain when one would want to select  the "assume additive
baseline" option when in the Study Function: ERSP.

This is my guess--the default ERSP baseline is (post-stimulus
uV^2)/mean_across_time(pre-stimulus uV^2). This is to compute change ratio.
This is why when we see 6dB increase in ERSP power, we say '200% power
compared with baseline'. This approach has a merit in avoiding
across-frequency normalization problem.

However, you can also take an average across conditions to compute baseline
value, assuming the baseline calculation is additive, just like ordinary
ERP baselines are. This approach does not make much sense if you think what
ratio you are computing there...

> After some searching we are still not clear on what this "additive"
option

It's not clear to me either.

> Specifically, we would like to know what is happening with the
baseline when we select, or don't select the additive baseline option?
And does it matter if our epochs already have a baseline created during
epoching
 (our baseline is the first second of the three seconds before the
trigger ).

If my guess is correct, then after doing it the mean baseline value in each
condition could be non-zero, while the mean of the mean baseline values
across condition should be zero. Personally I don't feel like testing it
because I don't see a point in doing it.

> ERSP always compares post trigger data to that of the baseline, right?

That's the default setting (all negative latency is counted as baseline).
But you can also specify the baseline period explicitly.

Makoto

On Tue, Jul 17, 2018 at 10:35 PM Kathleen Van Benthem <
kathy_vanbenthem at carleton.ca> wrote:

> Dear List,
>
> Can someone explain when
> ​one would want to
>
> ​select
>  the "assume additive baseline" option
> ​ when in the Study Function: ERSP​
> .
> After some searching we are still not clear on what this
> ​ "additive" option​
> does in the Stud
> ​y ERSP​
> function.
> We are interested comparing power in frequency bands in the few seconds
> before the "responded to" vs. the "ignored" triggers (auditory cues)
> ​ (N=30, two conditions per participant)​
> .
>
> ​Specifically, we would like to know what
> is happening with the
> ​baseline
>  when we select, or don't select the additive baseline option?   And does
> it matter if our epochs already have a baseline
> ​ created during epoching​
> (
> ​our baseline is ​
> the first second of the three seconds before the trigger
> ​​
> ).
>
> ERSP always compares post trigger data to that of the baseline, right?
> Thanks for your help in this matter,
>
> Kathleen Van Benthem
> kathy.vanbenthem at carleton.ca
>
>
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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