Dear SPM experts,
I have several questions concerning an er-fMRI design with parametric
modulation. I have a learning task and I want to look for modulations of
activity in time, using both 1st and 2nd order polynomial basis functions. I
am confused about the contrasts to use.
Say I have one condition A with a 2nd order parametric modulation: the first
three regressors in the design matrix will include the convolved onset
vector, the linear term, and the nonlinear term, respectively.
1. Is it correct to use a [0 1 1] F-contrast to look for areas that show
either linear OR nonlinear modulation of activity with time? Will the
t-contrast [0 1 0] show linear modulations alone, or would I have to set up a
separate model with only 1st order parametric modulation?
2. As far as I know, in a design with only 1st order parametric modulation and
one condition A, the t-contrast [0 1] will give increases and the t-contrast
[0 -1] will give decreases of BOLD activity in time.
In a design with 2nd order parametric modulation, the rationale for using a +1
rather than a -1 weight are not clear to me. Looking at plots of parametric
responses, I have noticed that a +1 t-contrast gives significant areas that
show an upright U-shaped curve, whereas a -1 t-contrast gives significant
areas that shown an inverse U-shaped curve.
Is this something that should be expected?
More in general, is there any way, with 2nd order parametric modulations, to
assess the directionality (shape) of the effects, in the same way as one does
with linear parametric modulations (+1 = signal increase, as opposed to -1 =
signal decrease)?
3. In my actual experiment I have two conditions A and B. I want to look for
areas whose activity is significantly more modulated in time in condition A
than in B. I set up a RFX model with a group of subjects in which I compare
the contrasts testing for [task A x time] interactions with contrasts testing
for [task B x time] interactions. Using a F-test to contrast the interactions
of task A with those of task B, I find a significant area X. The plot of
contrast estimates shows that the effect has a negative sign.
If I set up a separate RFX model including only [task A x time] interactions,
I find the same area X to be significantly modulated and, again, the plot of
contrast estimates shows that the effect has a negative sign (indicating an
inverse U shaped signal time course).
How are these results to be interpreted? As an evidence that:
a) the activity in area X is significantly more modulated in time in task B
than in task A.
b) the signal of area X in condition A has an inverse U shape and the effect
of condition A is more significant than condition B.
Extracting the plot of parametric response in area X averaged over all
subjects seem to support interpretation b), as condition A presents a
relatively pronounced inverse U-shaped response, whereas condition B presents
an 'almost flat' upright U-shaped response.
Sorry for the lengthy email.
Thank you for any advice.
All the best,
Marco
--
Marco Tettamanti, Ph.D.
Department of Neuroscience
Scientific Institute San Raffaele
c/o L.I.T.A. - room 25/5
Via Fratelli Cervi 93
I-20090 Segrate (MI)
Italy
Tel. +39-02-21717552
Fax +39-02-21717558
email: [log in to unmask]
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