Dear Vitaly,
> We are analysing Lyme Disease patients against group of normal
> controls. Because the differences in Lyme patients are
> subject-specific, we compare each patient individually against a group
> of controls. All scans are HMPAO SPECTs.
>
> For now, we are doing simple subtraction:
>
> Lyme Patient Control Group
> 1 1 -1
> 2 -1 1
>
> and proportional scaling is our global normalization method.
>
> The first contrast (1 -1) yields widespread change in activity
> throughout the whole brain. The second contrast (-1 1) shows a lot of
> change throughout the subcortical structures, but always excluding the
> cortex. These results hold for every individual comparison of a Lyme
> subject against the control group.
>
>
> We find these results mutually contradictory. If the first comparison
> looks for increases in flow in a Lyme patient, and the second one looks
> for decreases in flow in a Lyme patient, how can these two SPMs have
> extensive overlapping regions (in this case, in subcortical areas)? Do
> we interpret these subtractions correctly?
I suspect you are. I am sure that, on closer inspection, there is no
overlap between the two SPMs (even if the maximum intensity projections
give that impression). What you are seeing is presumably a systematic
cortico-subcortical change in Lymes disease. Because you have used
global normalization this represents a relative effect (e.g. the
absolute cortical activity could be normal but is high in relation
to subcortical activity).
I hope this helps - Karl
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