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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Hi, </span></font></div>
<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt"> If I understand you correctly, the following two papers may be of interest.
If you want to separate out the phase locked activity, and just look at the non-
phase locked activity, then Kalcher and Pfurtscheller (1995) suggest the
following procedure. </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Step 1: filter the individual EPOCHs to the band of interest. Step 2: average
the EPOCHs into a "filtered ERP". </span></font></div>
<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Step 3: subtract this average ERP from each individual EPOCH. </span></font></div>
<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Step 4: square these difference values </span></font></div>
<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Step 5: average the resulting squared deviations. </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Salmelin and Hari (1994) suggest an almost identical procedure, with the
only change being that in step 4 you simply take the absolute volage rather
than square the voltage (they call this temporal spectral evolution analysis). </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Both of these procedures assume that the ERP is unchanging from trial to
trial in both amplitude and time. If the ERP response varys, then the
subtraction of the grand average ERP will leave a residual component
behind. Although the squaring procedure of K & P would appear to magnify
this residual, the non-phase locked activity (which should be unaffected by
the subtraction) will be greatly increased. As such, the resulting "waveform"
will reflect the non-phase locked activity more than any residual ERP to a
greater extent than the absolute voltage method of S & H. </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Perhapse using a de-correlation procedure in step 3 (de-correlate the
EPOCHs and the ERP) would be more effective to deal with any trial by trial
variation in the individual ERP response. Of course, this would continue to
assume that there is no temporal variation in the ERP response (as do the
other procedures), but should remove the assumption of non-variation in
amplitude. I'm not aware of anyone using decorrelation rather than
subtraction, and there may be some concerns about this as a procedure (I'm
just thinking of it from reading your message). Anyway, hope these are of
some help. </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">- Jeff Hamm </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Kalcher, J. & Pfurtscheller, G. (1995). Discrimination between phase-locked
and non-phase-locked event-related EEG activity. <u>Electroencephalography &
Clinical Neurophysiology</u>, <u>94</u>, 381- 384. </span></font></div>
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<div align="left"><font face="Times New Roman" size="2"><span style="font-size:10pt">Salmelin, R. & Hari, R. (1994). Spatiotemporal characteristics of
sensiomotor MEG rhythms related to thumb movement. <u>Neuroscience</u>, <u>60</u>,
537-550.</span></font></div>
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