Just to add to Paul's and Ged's comments - if your hypothesis is about group
differences in lateralisation then what you need to do is a direct
statistical test of this, i.e. look for a group x hemisphere x task
condition interaction. This can't be done within SPM (unless you side-flip
images), so you would need to extract the regional values for each subject
and test them using standard statistical software.
I hope this helps
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
> Behalf Of Ged Ridgway
> Sent: 20 April 2007 10:31
> To: [log in to unmask]
> Subject: Re: [SPM] RFX question
> Hi Sophie,
> I agree with Paul. Another possibility might be if the right
> hemisphere activation was very variable in the controls (I have no
> idea how likely this is though), since this could firstly explain the
> lack of significance in the controls test, and secondly, due to a
> pooled variance estimate being used, the lack of a significant
> difference between groups. But I think Paul's answer is more likely
> the case, unless you check and find that actually the t-values for
> controls' right hemisphere are in fact well below the threshold.
> An interesting paper which talks about issues like this is Jernigan et
> al, 'More "mapping" in brain mapping...'
> Sophie Lafaille wrote:
> > using SPM2 I'm looking at some random effects analyses.
> > One sample T-test
> > when looking at task X within a group of 15 controls, we get very
> > significant left hemisphere activation with T-values of 10-14 (0.05 FWE)
> > when looking at task X within a group of 15 patients, we get very
> > significant bilateral activations with T-values of 9-15 (0.05 FWE)
> > when I enter all these con files into a 2 sample t-test to look at
> > controls vs. patients for task X I get nothing at a corrected level and
> > only start seeing the pattern of activation that makes sense at a 0.05
> > uncorrected
> > see attached, I've shown one sample data on top, and then the
> > subtraction on bottom.
> > am I doing something wrong or is the subtraction of either controls from
> > patients or vice-versa removing a lot of the right hemisphere activation
> > that I'm looking for and can only been seen at a much much lower
> > uncorrected p-value.
> > thank you, sophie
> > --
> > Sophie Josee Lafaille, B.Sc.(Hons.), M.Sc.
> > Research Officer II, Speech Fluency Laboratory 1059
> > Department of Speech-Language Pathology, University of Toronto,
> > 500 University Avenue, Rm 160, Toronto, Ontario, M5G 1V7
> > Phone: 416-946-8635 Fax:416-978-1596
> > We don't stop playing because we grow old, we grow old because we stop
> > playing.
> > - Anonymous