[Eeglablist] Difference Waves and IC Clusters

[Tarik S Bel-Bahar]

You should keep your trials with no probe in the data for ica.
Some groups have found their component of interest, then they remove the
other Ics and backproject just their component of interest, for advanced
single trial analyses. For your purposes, you may only need ica, and then
explore the dynamics of your expected p1 ic. As a side note, you will learn
something from exploring the decompositions with and without your probe
trials.
On Feb 28, 2012 6:32 PM, "Matthew Stief" <ms2272 at cornell.edu> wrote:

> Greetings Everyone,
>
> Is it possible (or sensible) to combine the difference wave and the ICA
> approach to isolating the component of interest?  That is, say I have an
> attention capturing stimulus and a probe, and I want to measure the P1 to
> the probe.  On some trials the probe doesn't appear, so of course one
> approach is to stay in ERPland and just subtract the activity of the trials
> where the probe did not appear from the trials when it did appear and
> measure the mean amplitude at one electrode.  But is it possible to do
> something similar on clusters of ICs instead of channels?  Is it possible
> to run ICA on datasets that each contain all relevant conditions, then
> cluster those ICs, choose a cluster that accounts for as much of the
> component of interest as possible, and then within that cluster subtract
> out the information coming from the trials where the probe does not
> appear?  Does that even make sense?  Would it be better to just toss out
> the no probe trials for ICA purposes so that the P1 elicited by the probe
> has a better chance of being isolated into a component?
>
> Perhaps some more detail and an appeal for more general advice is in
> order.  My experiment consists of a dot-probe task where the cues are a
> pair of photographs, one a nude male and one a nude female.  The
> photographs are rather large and appear on either side of the fixation
> cross at a moderate degree of eccentricity.  The cue appears for 100ms,
> followed by a 50ms gap, then a probe appears on either the left or the
> right side and persists until the participant presses a button indicating
> which side of the screen the probe appeared on.  On 1/3 of trials the probe
> appears on the left, on 1/3 it appears on the right, and on 1/3 it does not
> appear at all.  My challenge is to try to isolate out the P1 elicited by
> the isolateral probe from the ongoing response to the bilateral cue.  I of
> course would like to compare the condition where the probe is concordant
> with a male cue to the condition where the probe is concordant with a
> female cue.
>
> If I run ICA on a dataset that consists of all five conditions (Male
> Concordant L/R, Female Concordant L/R, No Probe), then in general I do not
> get a separate component that accounts for the left and right P1s elicited
> by the left and right probes.  If I try to separate the data into the left
> condition (Male Concordant Left, Female Concordant Left, No Probe) and
> right condition (Male Concordant Right, Female Concordant Right, No Probe)
> and run ICA on them separately I unfortunately fare little better.  Should
> I try perhaps try separating out the No Probe condition into its own
> dataset, in the hope that the probe P1 being more well represented in the
> data will improve its chances of being adequately represented by a
> component?  If I do that, does it make sense to run ICA on the No Probe
> dataset as well and then feed it into the clustering in order to pursue the
> difference wave approach I mentioned above?
>
> I am also unsure of what the optimal approach for selecting epoch limits
> and baselining is.  I have gathered that while more data is better for an
> ICA, for the purposes of isolating a small component like the P1 it may
> actually be advantageous to opt for less data so that the P1 is not swamped
> by the addition of irrelevant neural sources.  So I initially tried for a
> -250 to 1000 epoch, then a -250 to 500 epoch, and am now contemplating a
> -250 to 250 epoch.  I am simply removing the mean of the whole epoch before
> running ICA to eliminate drift and then removing a -250 to -150 baseline
> (i.e. to the onset of the cue) after ICA has been run.  Does that make
> sense?  Is it correct for me to limit my baseline to the onset of the cue
> rather than to the onset of the probe?  Since the cue is timelocked to the
> probe it seemed that I should.
>
> I am worried that if I both restrict the length of my epoch AND separate
> rather than concatenate my conditions then I will be rapidly approaching
> not having an adequate amount of data for a good ICA.  Luckily I have quite
> a bit of data for each subject, with about 350 trials for each condition.
> In terms of k*n^2, with the longest epoch length I'm considering and
> concatenating all conditions I have a k of almost 50, but with the
> restrictions I've been considering k is approaching 10 or 12 and I'm in
> danger of dipping down to 5 or 6.  I could always use PCA to try to find
> less components and since my research question doesn't really depend on the
> exact integrity of the overall EEG dynamics then perhaps it's fine, but I
> am not sure what the best approach is to reliably identify and measure the
> damned P1.
>
> Perhaps I just shouldn't be using ICA at all?
>
> I apologize for any amateurishness in what are probably pretty basic
> research design questions, I have somewhat had to teach myself these
> things, especially as they relate to actually carrying them out in EEGLAB.
> As always your patience and attention is greatly appreciated.
>
>
> --
> _________________________________________________________________
> Matthew Stief
> Human Development | Sex & Gender Lab | Cornell University
> http://www.human.cornell.edu/HD/sexgender
>
>
> Heterosexuality isn't normal, it's just common.
> -Dorothy Parker
>
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