> I would like to use VBM to compare the gray matter concentration and volume
> in two groups: Group A and Group B, where Group B is actually a
> control group. In order to study the correlation between the gray matter
> changes in both groups and visual acuity (va), I used the following
> settings:
>
> design type: single-subject: conditions & covariates
> select images
> condition: A A A A A A A A A A B B B B B B B B B B
> # covariates: 1
> covariate: spm_load (select the ASCII file with the values for visual
> acuity)
> covariate name: va
> interaction: none
> covariate centre: around overall mean
> global normalization: AnCova
> grand mean scaling: scaling of overall grand mean
> scale overall grand mean to: 100
> threshold masking: absolute
> analysis threshold: 25
> global calculation: user specified (select the ASCII file with the global
> values)
>
> it came up with 5 columns: 2 conditions + va + block + global, and I set
> the contrast as 0 0 1 0 0.
>
> Am I using the correct way to study such correlation? Then how should I
> interpret the results on the glass brains?
It depends what you want the statistics to relate to. The current model
will look for a significant relationship between GM and va that cannot be
explained by the subjects being in the patient group or not. Modelling
the control/patient effect as a confound in the model may be what you intend.
Best regards,
-John
--
Dr John Ashburner.
Functional Imaging Lab., 12 Queen Square, London WC1N 3BG, UK.
tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381
fax: +44 (0)20 78131420 http://www.fil.ion.ucl.ac.uk/~john
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