On Tue, Jul 17, 2012 at 5:47 AM, Glad Mihai <[log in to unmask]> wrote:
> Dear SPM fans,
>
> I have the following problem. In my design I use FIR covariates to find
> sequential activation in different areas of the brain from a motor and
> sensory task. I was inspired by the paper of Windischberger et al (Journal
> of neuroscience methods, 2008). Now, depending on how many covariates I
> include in the design matrix, I get a different type of activation. As far
> as I understand, this has got something to do with the degrees of freedom,
> which depends on the number of covariates.
>
> I have included three images as a comparison. The design matrix of each has
> the HRF model, a number of certain FIR covariates and the motion parameters
> at the end (+ constant).
>
> All three images are from the same subject; the TR is 514 ms.
>
> For 14_covariates.png I have used a total of 14 FIR covariates (1 to 15).
> Covariate 0 (not included in this design matrix) corresponds to the cue
> onset. The figure illustrates the activation for covariate #6. As you can
> see there is a very nice expected SMA activation.
>
> For 26_covariates.png I have used a total of... well 26 FIR covariates (0 to
> 25, 0 corresponds to cue onset). The contrast shows activation for covariate
> #6. The expected SMA activation is missing.
>
> For 1_covariate.png only one FIR covariate was used corresponding to
> covariate #6. I get no suprathreshold activation whatsoever.
>
> Why is there this difference? Furthermore, what would the optimal design
> matrix look like?
The regressors that you are contrasting are not the same. The two
without activation are white white black black and the one with
activity is black black white white. Thus, you are not comparing the
same thing.
I'd just use the standard approach to FIR in SPM, rather than creating
your own matrix.
>
> I'm looking forward to your answers.
>
> Regards,
> Glad
>
> --
> Glad MIHAI
> Funktionelle Bildgebung
> Uni-Klinikum Greifswald
> Walther-Rathenau-Straße 46
> 17489 Greifswald
> www.baltic-imaging-center.de
>
>
|