[Homer-users] Saving Data to Clipboard

[thuppert]

There are two reasons where this might occur (that I thought of off-hand).
I don't know if either applies.

1) By default, the averaging (or deconvolution) preforms a detrend on the
HRF.  This means that it removes linear trends from the data.  After
detrending, the change at the zero-point (time=0) is set to zero (this
removes any DC component).  If the 20-30s component is signifantly different
from zero mean, then this detrend can produce such results.  Ideally, the
HRF should return to baseline by the end of the post-stim time (and so no
problems should occur).  If not, maybe a longer post-time is required.

2) If you are doing a deconvolution, (particularly if any of the inter-stim
intervals are less then 30s) then this difference could be the result of the
20sec post-time not being long enough to fully model out the tail of the
HRF.  In that case, the longer post-time is giving a more accurate
deconvolution.  However, even if the HRF ends by 20s, still a 30s and 20s
deconvolution can differ depending on the efficancy of the stimulus
presentation (something set in the orignal experimental design).  The HRF
window (i.e. pre and post-stimulus times) are actually parameters that would
go into creating an ideal stimulus presentation for deconvolution.  At the
very least, think that a 30s deconvolution is fitting the same amount of
measurements with more unknowns (and hence affects the SNR).


*As a side note, I don't think that delta-conc. changes should really ever
be in the the range of 0.001.  That's a ~1mM change... plus/minus partial
volume etc. (which is like a 1000% change!!).


Ted Huppert, M.Sc.

PhD student-Harvard Univ.
Dept of Biophysics
Photon Migration Imaging lab
Mass General Hospital/CNY

Tele: (617)726-1223
Cell: (617) 869-1205

thuppert at nmr.mgh.harvard.edu



-----Original Message-----
From: homer-users-bounces at nmr.mgh.harvard.edu
[mailto:homer-users-bounces at nmr.mgh.harvard.edu]On Behalf Of Margaret
Duff
Sent: Monday, February 14, 2005 2:35 PM
To: homer-users at nmr.mgh.harvard.edu
Subject: [Homer-users] Saving Data to Clipboard


Hello,  I am saving on of my channel's data to the clipboard to be looked
at using Excel.  However, when I save data averaged over -5 to 20 seconds
and -5 to 30 seconds, the data changes.  I would expect the all the time
points from the -5 to 20 seconds average to remain in the -5 to 30 seconds
average data.  They are close but off by about .001 or so, which is kind
of a lot when looking at delta concentrations.  Is there anything inherent
to the averaging process or saving data to the clipboard that could cause
this discrepancy?  Thanks, Margaret Duff

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