Dear SPM list
I am confused a bit about choosing t-contrast weights on the first
level. Assuming having two conditions: contrasts [1 -1] and [10 -10]
produce the same t-value on the first level. However, when performing
a one sample t-test on a group of each contrast, the significance
differs (higher multiplier -> higher significance).
Maybe related to this:
When I load the con_NNNN.img images using spm_vol and spm_read_vols
the corresponding images seem to be equal, however other software
provides different gray level values for them.
So I am wondering how to set the weights in order to have valid second
level statistics? Which might be especially important when contrasts
must be balanced e.g. [2 -1 -1] vs [1 -0.5 -0.5] vs [0.5 -0.25 -0.25]
etc.
The versions I am using:
Matlab 7.7.0.471 (R2008b)
SPM8 3408
Thanks in advance to any help
Cheers
Christoph
--
Christoph Hofstetter, MSc in Biology
Laboratory for Neurology and Imaging of Cognition
Dept of Neurosciences
University Medical Center (CMU)
1 Michel-Servet - 1211 GENEVA - CH
http://labnic.unige.ch
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