Yes- this makes sense. I suspect it's probably noise from the lower SNR that's the issue. Thanks Michael!
Meera
________________________________________
From: Michael Erb [[log in to unmask]]
Sent: Thursday, March 31, 2011 3:55 PM
To: Meera Paleja
Cc: [log in to unmask]
Subject: Re: [SPM] "dalmation" pattern in data
Dear Meera,
your data look very noisy! You mentioned, that was a high-resolution
fMRI experiment, what is your actual voxel size? Remember that reducing
voxel size from 3x3x3 to 1.5x1.5x1.5 reduces the volume by 2x2x2=8. To
compensate this in term of SNR, you have to increase the measurement
time by 8x8=64. So instead of measuring 10 min you have to measure 10h
to get the same SNR!
Regards,
Michael.
Am 31.03.2011 19:20, schrieb Meera Paleja:
> Hi Alexander,
>
> I've attached the screenshot. This is what I'm seeing for single participants. I haven't done a group analysis yet since I only have 5 people in my sample so far. Does this effect normally disappear when I've done a 2nd level analysis?
>
> Meera
> ________________________________________
> From: Alexander Hammers [[log in to unmask]]
> Sent: Wednesday, March 30, 2011 4:50 PM
> To: Meera Paleja
> Cc: [log in to unmask]
> Subject: Re: [SPM] "dalmation" pattern in data
>
> Dear Meera,
>
>
> I may have missed the screenshot. What you describe sounds like what happens with very few degrees of freedom - unlikely to be a problem with fMRI, but how many did you have?
>
> ATB,
>
> Alexander
>
>
> -----------------------------
> Alexander Hammers, MD PhD
--
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Dr. Michael Erb
Sektion f. experimentelle Kernspinresonanz des ZNS
Abteilung Neuroradiologie, Universitaetsklinikum
Hoppe-Seyler_Str. 3, D-72076 Tuebingen
Tel.: +49(0)7071/2987753 priv. +49(0)7071/61559
Fax.: +49(0)7071/294371
e-mail: <[log in to unmask]>
www: http://www.medizin.uni-tuebingen.de/nrad/sektion/
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