Dear all,
Does anyone have any idea about the beta calculation in SPM? I found
it different with what I got from matlab regression.
I run SPM with a simple first level model with the design matrix
including two task condition, a linear trend term and a dc-term. No
global normalization and AR(1) are specified. After the estimation, I
can find the beta value for a example voxel, such as 0.5288.
I then take the time series of that voxel (Y), and do the regression
in matlab. The design matrix (or regressors X) has been set to be
exactly the same with SPM.xX.X. To find the beta value, I did the
calculation as (X' * X) \ X' * Y. However, the value I got is
-4.9671. I also considered the percentage unit, and did the
normalization with a factor of 100/dc-term. The result of that is
-0.6954.
Could anyone help me to understand the procedure I missed to find the
same regression value for the two cases?
Thanks in advance!
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Hongjian He
Zhejiang University, Hangzhou, China.
Email: hehongj(AT)gmail.com
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