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<div class="moz-cite-prefix">Dear Thomas, <br>
<br>
try using circ_dist.m (in the circ_stats toolbox by Berens). <br>
This should solve the circular difference issue.<br>
<br>
all the best, <br>
Tobias<br>
<br>
Am 15.01.2014 09:35, schrieb
<a class="moz-txt-link-abbreviated" href="mailto:Thomas.Baumgarten@med.uni-duesseldorf.de">Thomas.Baumgarten@med.uni-duesseldorf.de</a>:<br>
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<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">Dear
FieldTrip users,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">I am
working on a set of circular data (phase angles of ongoing
oscillations computed via Hilbert transform) and would like
to statistically compare two conditions (A,B). For this, I
use the circular statistics toolbox for matlab by P. Berens.
I worked on this problem from two different angles:
<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">1.
First, I tried to directly compare the two conditions via
the Watson-Williams two-sample test (function: circ_wwtest).
Unfortunately, this didn’t work out, since the test requires
an average resultant vector length of > 0.45 for n >=
11 entries/ subjects, an assumption which is not met by my
data.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">2.
Second, I tried to calculate the angle of difference between
the two conditions (angle(A) – angle(B)) and then used the
one-sample mean angle test (function: circ_mtest) to test if
the resulting angle of difference is significantly different
from zero. Here, the following problems arise: Since the
resulting angles for A and B range from –pi to +pi, there
are cases when the subtraction of the two angles results in
roughly +2pi or -2pi (e.g. cases where (A = pi) – (B = -pi)
= 2pi), resulting in an error from the circ_mtest function.
I tried to solve this problem by using a modulus (2pi)
operation (i.e. by ‘cleaning out’ the redundant
circumventions while at the same time preserving the angle
information), but unfortunately this didn’t work out either.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">The
only other option I can think of would be to generate
surrogate data (i.e. a matrix with the same dimensions as
the matrix with the angles of difference , only filled with
zeros) and to apply a cluster-based permutation test
(similar to ft_freqstatitics). Although this would take care
of my multiple-comparison problem, I am not quite sure if
the cluster correction is still valid in this case and if
this test would work for circular data.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">I
would greatly appreciate any comments and advice on this
matter.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">Thanks
for your help,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:12.0pt;line-height:115%" lang="EN-GB">Thomas<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;line-height:115%" lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;line-height:115%" lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"
style="margin-bottom:0cm;margin-bottom:.0001pt"><span
style="font-size:9.0pt;line-height:115%" lang="EN-GB">Thomas
Baumgarten, PhD Student<o:p></o:p></span></p>
<p class="MsoNormal"
style="margin-bottom:0cm;margin-bottom:.0001pt"><span
style="font-size:9.0pt;line-height:115%" lang="EN-GB">Institute
of Clinical Neuroscience and Medical Psychology, Medical
Faculty, Heinrich-Heine-University Düsseldorf,
Universitätsstraße 1, 40225 Düsseldorf, Germany</span><o:p></o:p></p>
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<br>
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<br>
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<br>
<br>
<pre class="moz-signature" cols="72">--
Dr. Tobias Staudigl
Fachbereich Psychologie - ZPR
Postfach ZPR
78457 Konstanz
ZPR, Haus 12
Tel.: +49 (0)7531 / 88 - 5703
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