Dear all,
I have a couple of "meta-technical" issues regarding correlations between
CBF and psychometric scores as they can be assessed in SPM.
We have done a H2O-PET study comparing subjects with and without
psychopharmacological stimulation (MDMA vs Placebo, in this case). We
looked at correlations between various psychological measures (mood,
perception etc) and rCBF using SPM co-variate analysis, both within Placebo
scans only, within MDMA scans only and pooling both Placebo and MDMA scans.
No significant correlations were found when correcting for multiple
comparisons, and we didn't have any a priori hypotheses, so there was no
rationale for not applying corrections for multiple testing.
Interestingly, however, we found that, when looking at uncorrected
p-values, CBF in the left amygdala correlated with anxiety scores within
MDMA scans, but also when pooling MDMA and Placebo scans. We thought that
the fact that this was the only regional correlation for which this was the
case may be significant. We included this finding in a manuscript and since
we did not want to report just any correlations that were significant based
on uncorrected statistics, we applied a systematic criterion regarding
which correlation results were reported: Correlations reported had to be
present *both* when
- including only MDMA scans into co-variate analysis and
- pooling MDMA and Placebo scans in the co-variate analysis
Of course, this criterion is data-driven and thus somewhat post hoc, but we
still think that it is a reasonable limitation in order to counteract the
lack of a correction for multiple comparison. We think that it is plausible
to assume that a true correlation between brain activity and some
psychological variable (basically a "neural correlate of consciousness")
should be present over a range of conditions, including placebo and
pharmacological stimulation. Thus, in our case, we think that it is not
unreasonable to require that it should be observable within MDMA scans AND
within the pooled data set including MDMA + Placebo scans.
Since we ran into some criticism from the reviewers, I'd like to ask you
for your opinion, particularly on these questions:
- do you think that the presence of the correlation between amygdalar CBF
and anxiety both within MDMA scans alone AND within the pooled data set
(MDMA+ Placebo), even if significant only using uncorrected statisticis, is
meaningful and has any informative value?
- if so, do you think the chosen criterion for counteracting the lack of
correction for multiple testing is reasonable or do you have any other
suggestions?
Thanks very much
Alex Gamma
University Hospital of Psychiatry
Research Unit
Lenggstr. 31
8029 Zurich
Switzerland
Email: [log in to unmask]
Phone: +41 1 384 26 32
Fax: +41 1 384 33 96
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