Statistical comparisons in virtual electrode spectrograms
Piers Cornelissen
p.cornelissen at PSYCH.YORK.AC.UK
Wed Nov 8 15:19:00 CET 2006
Dear all
Can anyone advise on the following:
I have 6 sets of virtual electrode data (from CTF based MEG recordings)
from a total of 9 subjects, each of whom carried out 4 conditions of
interest.
So far, for each virtual electrode I have succeeded (I hope) in computing
group level spectrograms for each condition, where each spectrogram
reflects either evoked or evoked+induced power changes.
What I would like to do is to a) compare spectrograms, and also b)compute
statistics for each group level spectrogram to ask the question whether
the power changes in the active window (0 - 700ms) are significantly
different from the those in the passive/baseline window (-250 - 0 ms).
So far I have had alot of help from Jan, and, on his suggestion, I have
copied out my query below, plus provided the matlab scripts which cause
the problem:
The attached script E_versus_EI.m shows the code for
analysing one condition from one particular virtual electrode. This gives
the evoked as well as the evoked+induced spectrograms (as far as I
understand it).
To carry out the comaprison between the evoked versus evoked+Induced test,
I thought I should be able to modify the freqgrandaverage step for
generating both iefgranfavg and efgrandavg by setting cfg.keepindividual
to 'yes'. In doing so, I was hoping that this would retain the 9 separate
powerspectra in each case, which it seems to have done. I was then hoping
that this would be sufficient input to the second script test_clusrand.m
(which is just a minor modification of E_versus_EL.m). Certainly the
powspctrm field for each dataset appears as: [4-D double] which I think is
correct.
However, when I ran clusterrandanalysis on iefgranfavg and efgrandavg, I
got the following errors:
>>
cfg15 =
statistic: 'depsamplesT'
alphathresh: 0.0500
makeclusters: 'yes'
minnbchan: 2
clusterteststat: 'maxsum'
onetwo: 'twosided'
alpha: 0.0500
nranddraws: 100
channel: {'V0'}
>> %cfg15.latency = [0 0.5] % increases sensitivity if prior temporal
information is knowm
>> cfg15.frequency = [5 40];
>>
>> %frange = 'all_freqs'; % file addition for saving
>>
>> [clusrand] = clusterrandanalysis (cfg15, ind_efgrandavg,
ind_iefgrandavg);
Selecting and formatting the data.
selected 1 channels
selected 191 time bins
selected 176 frequency bins
Calculating the neighbourhood structure of the channels.
??? Error using ==> clusterrandanalysis (getneighbgeometry)
Did not find gradiometer or electrode information.
Error in ==> /home/willis/plc/matlab/clusterrandanalysis.m
On line 473 ==> [cfg,data] = getneighbgeometry(cfg,data,varargin{1});
>>
I noticed also that the way I used freqgrandaverage in order to compile
the two datasets appeared to throw out the gradiometer information. At
least there was a warning to this effect. So, I am guessing that using
freqgrandaverage in this way is incorrect. Therefore please could anyone
suggest how I compile my 2 sets of nine spectrograms in such a way that
they provide appropriate input to clusterrandanalysis. Or, if as Jan seems
to suggest, that clusterrandanalysis can't cope with virtual electrode
data, does anyone have any other clever solutions.
Please!
Thanks in anticipation
Piers C
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