 |
 |
Dear Daniel, Would you be able to send me the SPM.mat corresponding to each of your analyses? When adding the globals as covariate manually, how did you compute them? Best regards, Guillaume. On 24/07/18 22:23, Daniel Martins wrote: > Dear all > > We have collected ASL data on drug and placebo administration at several time-points (different sessions), within-subject design (drug and time, both as within subject factors). For each time point i have run paired t tests between drug and placebo, using globals as a covariate (Ancova option) to identify drug effects on cerebral blood flow. I have also run some analysis where i calculated difference images first (drug-placebo) for each time point and then have run one-sample t test (controlling for globals of the difference images in the same way), where i was expecting to get similar results to the ones achieved in the paired t tests. > > Surprisingly, i could not achieve similar results between paired t tests and one-sample t test (on the difference images), unless i enter the globals as a separate covariate without mean centering. Results between paired and one sample t test on the difference images are exactly the same when not controlling for globals. Adding manually the globals as a covariate, with or without mean centering, do not change the results of the paired t test. However, i got very strange results on the one sample t tests when i ask spm to calculate and add globals as a nuisance variable (using an explicit grey matter mask), when i add globals manually in the user option for globals (both with spm centering the covariate to the mean automatically) or when i add a covariate manually with mean centering. If i enter the globals manually as a covariate and do not ask for overall mean centering, then i achieve results similar to the ones i got in the paired t tests. > > Did someone come across with the same issue or may advice on the use of centering to the mean in difference images? > > Thanks in advance > > BW > -- Guillaume Flandin, PhD Wellcome Centre for Human Neuroimaging University College London 12 Queen Square London WC1N 3BG