Dear list subscribers
This is a question from a novice and uneducated SPM user:
I am conducting the analysis of a dataset comparing cerebral glucose
metabolic differences between depressed and non-depressed adults.
It is widely accepted that in depressed individuals metabolic
reductions in the dorsolateral prefrontal cortex will occur. These
reductions might be added by reductions in other sites, for example
in the parietal lobe. Accordingly the hypothesis in this project
is that metabolism in the dorsolateral prefrontal cortex will be
significantly lower in depressed than in normal controls.
SPM 99 was utilized in this comparison between the two groups, one
of FDG PET images from 19 depressed individuals and the other group
FDG PET images from 13 age matched normal controls.
In setting up the analysis the design selected was a t-test for
comparison between the two groups. In defining the t-tests and
new contrast we entered controls > patients or 1 -1 and
subsequently patients>controls or -1 1.
I am left with two T maps and cannot quite understand what they
communicate. A few questions come to mind:
1 – when contrasts between groups or conditions are defined, e.g.,
something > something else, is > asked in what respect: voxel
intensity?
2 – If the main question relatively to the biology underlying the
PET studies relates to metabolic reductions, are the design and
contrast specified above appropriate to create the model that might
answer the question being asked ?
3 – How should the T maps be read/interpreted? e.g., if indeed there
are metabolic reductions in the depressed group, how should I
expect the T-map to look like? Shall I look for those reductions
as clusters, or as areas without clusters? Shall I look at
controls- patients or patients-controls, or both?
I would very much appreciate any attempts at answering to my query.
I apologize in advance for any inconsistencies in the information
provided or in the phrasing of my question.
Best wishes
Salomao (Solomon) Segal, MD
Department of Psychiatry
Columbia University
1051 Riverside Drive
New York, NY 10032
212 543 5382
212 543 6630 (Fax)
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