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Dear Thomas,<br>
<br>
This is indeed the approach that I have followed succesfully a couple
of times (e.g. Bastiaansen et al., JOCN 2006), although the terminology
that you are using is somewhat confusing. I (and I guess most people)
would refer to induced activity as that part of the EEG that is
non-phase-locked, so I would restate your equation to:<br>
induced = EEG - evoked.<br>
<br>
However, there is a drawback to this approach, since it assumes that
the ERP is absolutely stationary over trials. This is not the case in
reality (e.g. subjects' attentional level or other states may change
from trial to trial, giving rise to variability in the single-trial
ERPs). This means that by subtracting the average ERP, one may
introduce frequency components in the residual EEG that were not
present before. Klimesch, and Kalcher and Pfurtscheller, have come up
with ways of scaling the average ERP so as to yield a best fit of the
average with each single-trial ERP, but also that approach may be
sub-optimal.<br>
My latest way around the problem is to run a TF analysis on the
untreated EEG (containing both evoked and induced activity), and
comparing this to a TF analysis of the subject-averaged ERPs (the
evoked activity alone). Qualitative differences between the two
analyses can now only be attributed to induced activity.<br>
<br>
Marcel<br>
<br>
Thomas Thesen wrote:
<blockquote cite="mid001d01c7568f$4b0d1890$0202fea9@D3K61L91"
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<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Hi
FieldTrippers,<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Following up
on this conversation. It
seems that the ‘induced activity’ contains both phase-locked and
non-phase-locked activity, whereby the ‘evoked’ activity contains
only phase-locked activity. Is it then kosher to separate these
components by
linear subtraction? For example, if we first compute the ‘induced’
activity by averaging power over individual trials, and from that
subtract the
‘evoked activity’ (calculated based on average response) to get the
induced activity without any phase-locked activity?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">So if <o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Induced =
Phase + Non-Phase<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">And <o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Evoked =
Phase<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Then <o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Non-Phase =
Induced – Evoked<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Or does the
fact that this is a linear
operations on data that have been constructed through a non-linear
process
render this somehow invalid? It has certainly been done before. Your
comments
would be much appreciated.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Cheers,<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;">Thomas<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Arial" size="2"><span
style="font-size: 10pt; font-family: Arial; color: navy;"><o:p> </o:p></span></font></p>
<div
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<div>
<div class="MsoNormal" style="text-align: center;" align="center"><font
face="Times New Roman" size="3"><span style="font-size: 12pt;">
<hr tabindex="-1" align="center" size="3" width="100%"></span></font></div>
<p class="MsoNormal"><b><font face="Tahoma" size="2"><span
style="font-size: 10pt; font-family: Tahoma; font-weight: bold;">From:</span></font></b><font
face="Tahoma" size="2"><span
style="font-size: 10pt; font-family: Tahoma;"> FieldTrip
discussion list [mailto:<st1:PersonName w:st="on"><a class="moz-txt-link-abbreviated" href="mailto:FIELDTRIP@NIC.SURFNET.NL">FIELDTRIP@NIC.SURFNET.NL</a></st1:PersonName>]
<b><span style="font-weight: bold;">On Behalf Of </span></b>Christian
Hesse<br>
<b><span style="font-weight: bold;">Sent:</span></b> Thursday,
February 22, 2007
3:41 AM<br>
<b><span style="font-weight: bold;">To:</span></b> <st1:PersonName
w:st="on"><a class="moz-txt-link-abbreviated" href="mailto:FIELDTRIP@NIC.SURFNET.NL">FIELDTRIP@NIC.SURFNET.NL</a></st1:PersonName><br>
<b><span style="font-weight: bold;">Subject:</span></b> Re:
[FIELDTRIP]
Estimating the power in EEG frequency bands</span></font><o:p></o:p></p>
</div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">Hi Shantanu,<o:p></o:p></span></font></p>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
<div>
<div>
<blockquote style="margin-top: 5pt; margin-bottom: 5pt;" type="cite">
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">When I want to analyze the power in different
eeg frequency bands
(alpha,<span class="apple-converted-space"> </span><o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">beta, etc), do I use a grand average for all
subjects, or do i take
only<span class="apple-converted-space"> </span><o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">data trial by trial for all electrodes,
estimate the powers, and then<span class="apple-converted-space"> </span><o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">average them for all trials?<o:p></o:p></span></font></p>
</div>
</blockquote>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">You use the the latter approach, averaging
the power over trials
(subject average) and then over subjects (grand average) if you wish to
look at
INDUCED activity, i.e. task or event related modulation in the
amplitude
time-course of ongoing oscillatory activity (this analysis is not
sensitive to
the phase differences of the oscillations on different trials).<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">You can also average the signals first (in
the time domain) and then
compute the power spectrum, in which case you will see the spectral
properties
of EVOKED activity, i.e., activity that is phase or time locked to the
event /
stimulus / response.<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">Both of these time frequency visualizations
give information that is
complementary to looking a the time course of your event-related or
evoked
potentials (for EEG) or fields (for MEG) which you get by doing a
time-locked
average of the raw signals.<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">Hope this helps to clarify,<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;">Christian<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
<div><span
style="border-spacing: 0px; orphans: 2; widows: 2; word-spacing: 0px;"><span
style="border-spacing: 0px; orphans: 2; widows: 2; word-spacing: 0px;">
<div>
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style="border-spacing: 0px; orphans: 2; widows: 2; word-spacing: 0px;"><span
style="border-spacing: 0px; orphans: 2; widows: 2; word-spacing: 0px;"><span
style="border-spacing: 0px; orphans: 2; widows: 2; word-spacing: 0px;">----------------------------------------------------------------------<o:p></o:p></span></span></span></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">Christian
Hesse, PhD,
MIEEE<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;"><o:p> </o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">F.C.
Donders Centre
for Cognitive Neuroimaging <o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><st1:address w:st="on"><st1:Street w:st="on"><font
color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">P.O.
Box</span></font></st1:Street><font color="black" face="Helvetica"
size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;"> 9101</span></font></st1:address><font
color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;"> <o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">NL-6500
HB Nijmegen <o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">The <st1:country-region
w:st="on"><st1:place w:st="on">Netherlands</st1:place></st1:country-region><o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;"><o:p> </o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">Tel.:
+31 (0)24 36
68293<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">Fax:
+31 (0)24 36
10989<o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;"><o:p> </o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">Email:
<a href="mailto:c.hesse@fcdonders.ru.nl">c.hesse@fcdonders.ru.nl</a><o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">Web: <a
href="http://www.fcdonders.ru.nl">www.fcdonders.ru.nl</a><o:p></o:p></span></font></p>
</div>
<div>
<p class="MsoNormal"><font color="black" face="Helvetica" size="1"><span
style="font-size: 6.5pt; font-family: Helvetica; color: black;">----------------------------------------------------------------------<o:p></o:p></span></font></p>
</div>
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style="font-size: 6.5pt; font-family: Helvetica; color: black;"><o:p> </o:p></span></font></p>
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style="font-size: 6.5pt; font-family: Helvetica; color: black;"><o:p> </o:p></span></font></p>
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<br>
</span></span></font><o:p></o:p></p>
</span></span></div>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size: 12pt;"><o:p> </o:p></span></font></p>
</div>
</div>
</div>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
dr. Marcel C.M. Bastiaansen.
Max Planck Institute for Psycholinguistics
Visiting Adress: Wundtlaan 1, 6525 XD Nijmegen, the Netherlands
Mailing adress: P.O. Box 310, 6500 AH Nijmegen, the Netherlands
phone: +31 24 3521 347
fax: +31 24 3521 213
mail: <a class="moz-txt-link-abbreviated" href="mailto:marcel.bastiaansen@mpi.nl">marcel.bastiaansen@mpi.nl</a>
web: <a class="moz-txt-link-freetext" href="http://www.mpi.nl/Members/MarcelBastiaansen">http://www.mpi.nl/Members/MarcelBastiaansen</a>
and
FC Donders Centre for Cognitive Neuroimaging
Visiting address: Kapittelweg 29, 6525 EN Nijmegen, the Netherlands
Mailing address: PO Box 9101, 6500 HB Nijmegen, the Netherlands
phone: + 31 24 3610 882
fax: + 31 24 3610 989
mail: <a class="moz-txt-link-abbreviated" href="mailto:marcel.bastiaansen@fcdonders.ru.nl">marcel.bastiaansen@fcdonders.ru.nl</a>
web: <a class="moz-txt-link-freetext" href="http://www.ru.nl/aspx/get.aspx?xdl=/views/run/xdl/page&ItmIdt=20592&SitIdt=119&VarIdt=96">http://www.ru.nl/aspx/get.aspx?xdl=/views/run/xdl/page&ItmIdt=20592&SitIdt=119&VarIdt=96</a>
--</pre>
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