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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'>Dear Rodolphe and Michael,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US>strictly
speaking cluster analysis (as a construct to get a univariate test instead of a
mass-univariate one) only tells you that there IS a significant cluster. </span>The
exact shape and temporal evolution of this cluster is not part of what's tested
- as least as far as I understand this issue. That is a data point can surpass
your cluster inclusion threshold by chance - a portion of alpha data points
will actually do this. In addition, these point can be accidentally close to
other clustermembers. Hence, it will be included in a cluster. So 'true'
cluster membership of a certain electrode/timepoint/frequency is not
statistically 'guaranteed'. Additional tests would be necessary.<br>
<span lang=EN-US>Then one thing that this argument does not account for is that
'accidentally neighbouring a cluster' is not so likely because of the
smoothness of MEEG data - but I do not know at the moment how you could exploit
this.<br>
<br>
<br>
<span style='color:#1F497D'><o:p></o:p></span></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'>This
is well formulated. The null hypothesis that is tested by the cluster-based
permutation tests involves that the spatiotemporal biological data in the
experimental conditions that are being compared are drawn from the same
probability distribution. They are NOT testing null hypotheses of the following
type: there is no difference between the conditions at channel A and time point
T whereas there may be differences at other (channel, time point) pairs. In
fact, such null hypotheses do not make much sense when dealing with
electrophysiological data because (a) because of volume conduction, spatially localized
effects are not plausible, and (2) because neurophysiological processes are not
instantaneous, highly temporally localized effect are not plausible either.<o:p></o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'>Best,<o:p></o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'>Eric
Maris<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Verdana","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US
style='font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><span lang=EN-US>Michael</span><span
lang=EN-US style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'><o:p></o:p></span></p>
<div class=MsoNormal align=center style='text-align:center'><span
style='font-size:9.0pt;font-family:"Verdana","sans-serif";color:black'>
<hr size=3 width="100%" align=center>
</span></div>
<p class=MsoNormal><b><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'>Von:</span></b><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'> Rodolphe <batrod@GMAIL.COM><br>
<b>Gesendet:</b> Oct 20, 2010 12:49:06 AM<br>
<b>An:</b> FIELDTRIP@NIC.SURFNET.NL<br>
<b>Betreff:</b> [FIELDTRIP] Cluster change over time<br>
<br>
Dear Fieldtrip users, <o:p></o:p></span></p>
<div>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'>I used the Monte-Carlo method with cluster correction to compare
two conditions within the same group of subjects.<o:p></o:p></span></p>
</div>
<div>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'>I didnt average over time, thus the clusterplot showed several
topographic plot of the cluster that was found to be significant.<o:p></o:p></span></p>
</div>
<div>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'>I can see that this cluster evolves over time (number and place of
significant electrodes highlighted). My question is, considering the
statistical method using for this analysis, can i say that this change over
time is significant or does it require further analysis like ANOVAS in
concerned electrodes?<o:p></o:p></span></p>
</div>
<div>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'>Thanks a lot, <o:p></o:p></span></p>
</div>
<div>
<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Verdana","sans-serif";
color:black'>Rodolphe N., Ph.D.<o:p></o:p></span></p>
</div>
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