They should be identical. Did you change any of the default values?
Did you double check that the order of the pairs is the same for each
subject?
SPM.xY.P(:) will show you the order of the files.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Thu, Nov 8, 2012 at 8:49 AM, Zhenhao Shi <[log in to unmask]> wrote:
> Dear SPMers,
>
> I have a question regarding the difference between one-sample t-test and
> paired t-test in SPM8.
>
> I got one single group of 20 subjects with Condition A, Condition B, and a
> Neutral baseline, and I would like to compare A and B. In the first level, I
> did three T-contrasts for each subject: "A > Neutral", "B > Neutral", and "A
>> B", which resulted in three contrast images: con_0001.img, con_0002.img,
> con_0003.img. In the second level, I did a one-sample t-test using the files
> of con_0003.img from all subjects, and entered [1] for a t-contrast, which
> gave me reasonable brain activations for A > B. After that, just out of
> curiosity, I tried paired t-test and entered con_0001.img and con_0002.img
> in pairs each subject at a time. The design matrix looks like that the first
> two columns are of my interest and the next 20 columns are for each subject.
> I then entered [1 -1] for a t-contrast, but obtained totally different
> results from those in the previous one-sample t-test. From my understanding,
> the paired t-test on A>Neutral and B>Neutral should be identical to the
> one-sample t-test on A>B, right?
>
> To figure out what went wrong, I tried using spm_imcalc_ui to manually
> substract con_0002.img from con_0001.img for each subject, and got another
> 20 new image files I named as con_1vs2.img. Using MriCroN, I see that
> con_0003.img and con_1vs2.img have the same voxel values. So, it looks as if
> my first level analysis was right. Then could the different results of
> one-sample t-test and paired t-test be caused by their different algorithms?
> Or there must be some mistakes I managed not to notice?
>
> Looking forward to you experts' reply. Thanks in advance!
>
> Best,
> Z
>
>
> -----
>
> Zhenhao SHI
> Culture and Social Cognitive Neuroscience Lab
> Department of Psychology
> Peking University
> 5 Yiheyuan Road
> Beijing 100871, P.R.China
> Email: [log in to unmask]
> http://www.psy.pku.edu.cn/LABS/CSCN_lab
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