tomh at KURAGE.NIMH.NIH.GOV
Fri Oct 22 18:25:17 CEST 2004
> Sounds interesting. But why are you running an ANOVA? The coherence in
> a voxel will not be normally distributed.
You'd think that, wouldn't you? :-)
[By the way, that's an American expression meaning roughly, I'm
just as surprised as you are.]
Here's a random sample, from subject 6, condition 4:
#Magnitude Freq Cum_Freq
-2.124902 21 21
-1.874683 47 68
-1.624463 74 142
-1.374243 134 276
-1.124023 180 456
-0.873804 208 664
-0.623584 249 913
-0.373364 323 1236
-0.123144 2052 3288
0.127076 343 3631
0.377295 264 3895
0.627515 219 4114
0.877735 161 4275
1.127955 114 4389
1.378175 72 4461
1.628394 47 4508
1.878614 33 4541
2.128834 16 4557
2.379054 2 4559
2.629274 1 4560
These coherence values have already been z-scored. That's
something I forgot to mention before -- since the coherence
values turn out to be not too badly distributed, and since I'm
only interested in the differences anyway, I z-scored 'em.
> Furthermore, it does not allow for a decent multiple comparison
> correction (except bonferoni), since GRFT does not apply.
That's true, but my experience has been that normalizing the
distributions (so that individual differences in power levels
between subjects are eliminated) has almost exactly the same
effect as a random permutation analysis (which I have done, but
not on these data). The beamformers apparently provide quite a
lot of independence already (you know, it's all coming from the
same CSD matrix, but each voxel has a large prior in the form of
the forward solution -- somebody should write a paper about
that). And since the coherence is normalized anyway, it's fine.
I can always do a permutation analysis later, by shuffling the
conditions and rerunning the ANOVAs a few thousand times (they
are very fast).
> Did you try sourcestatistics with method randomization or
> randcluster? They do multiple comparison correction.
You mean sourcedescriptives? No, I haven't, but I will. I was
in a hurry (Neuroscience is next week, you know). Did I mention
it took 24 hours to calculate all the volumes? (Yes, I know, use
the Beowulf cluster. NIH's cluster is called "Biowulf". Ha ha.)
> Sofar no one ever asker me for the phase, but it would be interesting
> to do some more postprocessing of the source.avg.csd matrices to make
> something like phase easier to plot. I would like to hear suggestions.
Let me think about that for a while.
Dr. Tom Holroyd
"A man of genius makes no mistakes. His errors are volitional and
are the portals of discovery." -- James Joyce
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