See inline responses below.
On Mon, Apr 2, 2012 at 10:55 PM, xiuqin jia <[log in to unmask]> wrote:
> Dear experts,
>
> Here, I have two question about Beta value. I have searched it in the SPM
> archive, but it seems I cannot understand it completely. So I post it here.
>
>
> 1) Is there a plausible range of beta value?
There are several issues with this question. (1) beta values refer to
the estimates of each column of your design matrix; (2) when people
report "beta" they are usually referring to their contrast; (3) it is
also dependent on the scaling of the column; (4) it is dependent on
the scaling of the data (e.g. in SPM the data are scaled to 1000, I
believe).
Because every column has a beta estimate, there isn't a specific
range. The beta's for tasks are usually very low (in the range of a
few percent). The beta for the constant is dependent on the scanner,
patient position, blood flow, scaling, among other factors. In papers,
you have to consider how the contrast was designed:
(a) 1s over the columns -- sum of the betas
(b) 1/N over the columns -- mean of the betas
>
> In my experiment, there are 4 conditions: SI, CI, SA, CA. I extracted the
> beta values of the 4 conditions in bilateral DLPFC (dorsolateral prefrontal
> cortex) and PPC (posterior parietal cortext). However, It seems a bit larger
> than ones in the published papers. I have attached the figure.
>
> I'm not sure, whether is it plausible?
What was your contrast vector?
>
> 2) Does the negative beta value indicate deactivation?
Yes. Less activity than baseline.
>
> The positive beta value can be understood as more active, can the negative
> beta value be understood as negative activation? How to explain the negative
> beta value?
>
>
> Any comments are deeply appreciated. Thanks a million!
>
> Best Regards,
>
>
> xiuqin jia
>
>
>
> -----------------------------------------------
>
> Xiuqin Jia, Ph.D Candidate
> Department of Radiology,
> Xuanwu Hospital,
> Capital Medial University,
> 45 Changchun Street,
> Xuanwu District,
> Beijing, 100053 China
> Tel: (+086)-138-1012-5607
>
>
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