Dear Cristian,
As far as I know, individual t-images are not used for second level analysis.
Normally you include in second level individual con (contrast) images.
One sample t-test can be used for estimation of the effect of the condition withing group. Two sample t-test - for comparison for this effect between two groups.
So, in order to explain both extraordinary high t sore for control subjects and the absence of difference between control and patient group i think it is better to review intensities of con images in your area of interest.
A possible explanation of your resuts can be high variation of the effect in patient group and low variation in control group.
Good luck!
Vladimir
Volodymyr B. Bogdanov, PhD
--- On Thu, 4/4/13, Christian R. <[log in to unmask]> wrote:
> From: Christian R. <[log in to unmask]>
> Subject: [SPM] 2nd Level Analysis & T-value
> To: [log in to unmask]
> Date: Thursday, April 4, 2013, 9:15 AM
> Dear SPM experts,
>
> I’m a SPM beginner, doing a fMRI study with two groups (19
> patients & 19 controls). I am using a block design with
> three different conditions (condition1, condition2 and
> rest). Right now, I’m doing the 2nd level analyses and I
> have met a problem that is quite confusing me.
>
> First, I have done the within group comparisons (FWE
> p=0.05). For the comparison condition1 vs. rest the controls
> show a very high t-value, T=17.44, in one special region
> (Temporal Pole Sup left; -48 8 -2; cluster size 525 voxels).
> However, the patients show absolutely no activation in this
> region. So, I have expected, that if the controls have such
> a high t-value whereas the patients show no correspondent
> t-value for the same area, the between group comparison
> controls vs. patients has to produce a significant result in
> this area, indicating a much higher activation for the
> controls. But the between group comparison - FWE corrected
> as well as uncorrected! - shows absolutely no group
> differences in this area. This is not what I have expected
> and I can’t explain this result. I hope that someone of
> you has an idea…
>
> A further question refers to the above mentioned t-value.
> The t-value T=17.44 seems to be extraordinary high to me –
> especially compared to all the other t-values in my
> analyses. So, I checked the t-values for every single
> control subject (n=19) for this special region, in order to
> get an explanation for this very high t-value. The result
> was quite confusing, because the single subjects only showed
> t-values ranging from -0,22945 to 1,05096. This range
> doesn't explain T=17.44 to me. I hope, that someone of you
> has an explanation for that high t-value…
>
> Thanks in advance for any help you provide!
> Christian
>
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