Preproc - thresholding negative z-scores too?

[jan-mathijs schoffelen]

Dear Alex,

As far as I remember, the default settings for artifact_eog (the
'higher' level function which calls artifact_zvalue later on) contain
the (absolute of the) hilbert transform.
The hilbert-transform of a bandpass filtered signal is an estimate of
the instantaneous amplitude. Since it is an absolute value, it is
always positive and should reverse
the polarity of the EOG, as would rectification.
I do not really understand your statement that the polarity is not
changed by either rectification of hilbert transformation. Could it
be that you specified your options in
cfg.artfctdef.eog, and then called artifact_zvalue? or
cfg.artfctdef.zvalue and called artifact_eog?

I guess specifying cfg.artfctdef.XXX. rectify/hilbert = 'yes', and
subsequently calling artifact_XXX (eog or zvalue) should do the
trick. Of course, specifying cfg.artfctdef.XXX.feedback = 'yes',
gives you an idea how the data look.

Good luck

Jan-Mathijs


On Oct 27, 2008, at 3:04 PM, Alexander Maye wrote:

> Dear List!
>
> In my dataset I have a nice EOG signal, in which the respective
> artifact detection routines
> nonetheless do not detect artifacts,since its polarity is negative.
> Consequently the
> z-scores also deviate towards negative, and are always below (the
> positive) threshold
> value. So, shouldn't artifact_zvalue threshold the *absolute* z-
> value instead of the raw
> z-score? I know this has been asked last year, but couldn't find an
> answer. Any
> opinions/preferred solutions? Ah, and yes, I played with hilbert
> and rectify, but hilbert does
> not change the polarity (what does it anyway?), and rectify
> rectifies the data, not the
> z-scores.
>
> Best,
>
>                        ALEX.
>
>
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The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.