I have been running the higher level analysis of my simplest experiment
now since Sunday morning. It looks like it will still require a few
more days of processing to be finished (I am using an 867MHz G4 with
512MB Ram.) As I have looked at the initial results for my simple
contrasts (W, B, N,P), I am getting clusters that contain 20,000
voxels. For instance:
Cluster List
Cluster Index Voxels P -log10(P) Max Z x (mm) y (mm) z (mm)
2 20330 0 48.6 8.06 -30 -92 4
1 675 0.000699 3.16 5.23 26 20 26
I ran this analysis using the default settings. Clearly, these results
are not terribly useful for analysis. I ran this using the default
settings -- cluster thresholding, 2.3, .001.
I am having exactly the opposite problems for the more complex
contrasts (W>B, B>W, N>P, P>N) and am getting no activations.
Since this first group analysis is just an exploratory run to help me
learn to run FSL, it isn't a big deal that I'm getting these difficult
to manage results. However, I am going to be doing my next analysis
under more time pressure and it is a substantially more complex
experiment.
How should I go about rethresholding my runs? Especially, since it
appears I am having two opposing problems. With the simple contrasts,
(W, B, N, P) my understanding is that I should be raising the cluster
thresholding, right? But, which of the two numbers should I be raising
to get interpretable contrasts? Or, should I just switch to
voxel-based clustering? Or, something else?
Also, what should I do about the more complex contrasts (W>B, B>W, N>P,
P>N) in order to see some meaningful activations?
And, is there a way to simultaneously achieve these seemingly opposing
goals? Or, do I rerun the analysis with higher thresholds to get
meaning out of the simple contrasts and lower thresholds with the more
complex contrasts?
I am not anxious to meander through the parameter space to find
reasonable settings for the threshold values since there is such a
long time requirement for my data to run. So the wisdom of experience
would be especially critical. Also, since I have more pressing time
problems for my next experiment, any advice on thinking about it?
Should I use the results that I am generating right now as masks for
the next run? Would that make sense as a solution?
Darren
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