Claus -
I then used an F-contrast (1 0 0; 0 1 0; 0 0 1)
testing for whether
any of the three regressors explains some variance in
my data.
In addition, I calculated a T-contrast (1 0 0) for the canonical
hrf only.
In general, the pattern of results looks very similar for the
two
contrasts. However, what I do not understand is why the t-contrast
shows some voxels
and clusters which fail to be significant at all in the
F-contrast.
The reason is
that, if the majority of variance is captured by the canonical HRF,
then the T-test is a more sensitive test of this (it tests only one
dimension of the subspace tested by the F-test, including a specific
direction - ie one-tailed). As you say, if variance loaded on the derivatives,
you could find the opposite pattern of significance in the F but not T
map.
In
many conditions, subjects and brain regions, the canonical HRF does seem
the capture the majority of variance. However, there are some
regions/subjects/conditions where the derivatives capture additional variance,
see, eg, section 3.1 of:
Rik
----------------------------------------
Dr
Richard Henson
MRC Cognition & Brain Sciences Unit
15 Chaucer
Road
Cambridge
CB2 2EF, UK
Tel: +44 (0)1223 355 294 x522
Fax: +44 (0)1223 359 062
----- Original Message -----
Sent: Monday, October 10, 2005 1:39
PM
Subject: [SPM] t-test on hrf for model
with derivatives
Dear SPM community,
I recently reanalyzed data I had
modeled before using the hrf only, this
time using a model with
hrf+temporal and dispersion derivative.
According to the suggestions by
Rick Henson (e.g.
ftp://ftp.fil.ion.ucl.ac.uk/spm/data/rfx-multiple/rfx-multiple.htm)
I set up
a rfx 2nd level analysis for this.
I then used an
F-contrast (1 0 0; 0 1 0; 0 0 1) testing for whether
any of the three
regressors explains some variance in my data.
In addition, I
calculated a T-contrast (1 0 0) for the canonical hrf only.
In general,
the pattern of results looks very similar for the two
contrasts.
However, what I do not understand is why the t-contrast
shows some voxels
and clusters which fail to be significant at all in the
F-contrast.
Shouldn't it be the other way round (if at all)? I.e. that
I see activation
in the F contrast which I do not see in the T contrast
(with the reason for
this being that the F test also uses variance
explained by the derivatives)?
Thx a lot for helping me with
this
claus