I'm struggling with the use of parametric modulation in my design. I
have 3 conditions (stimuli), the response to which I want to modulate
with a subjective rating that was obtained after every stimulus (6
ratings per stimulus per session).
I have two similar questions:
1) If my parameter values do not change during the session (which
happens sometimes for one of the stimuli), SPM leaves out the
'modulation column(s)' (admittedly, there is nothing to modulate). I
guess this is a feature, but it is inconvenient for generating multiple
batch jobs because it changes the number of columns in the design
matrix, rendering my pre-specified contrasts useless. Could this be
turned off somehow?
2) Essentially the same thing occurs for higher order modulation: a 2nd
order modulation column is only included when the parameter value
changes at least twice. I understand that for fitting a quadratic curve
rather than a line one needs more than 2 different parameter values,
however, I would still want to have one column for 1st order modulation
(by 'parameter' ) and one for 2nd order modulation (by 'parameter
squared'), regardless of my parameter values. This might reflect my lack
of understanding of how parametric modulation is performed in SPM but,
could this be done somehow?
Thank you, regards,
PS I plan to do an analysis with all three stimuli merged into one
condition; then I will have enough variation in my parameter values not
to encounter the issues mentioned above.
Paul A.M. Smeets, PhD
Image Sciences Institute
University Medical Center Utrecht
Heidelberglaan 100, Rm Q0S.459
3584 CX Utrecht, The Netherlands
Tel: +31 88 75 56682
Fax: +31 30 - 251 3399