Dear SPM users,

we are analysing an event-related fMRI paradigm - four different trial
types are encoded in the
first level design matrix, two of those are compared against each other
using t-tests.

We would now like to attribute reaction times to each single trial,
assuming that there is a linear
relationship between BOLD amplitude and faster reaction times (for a
start). We are still interested in the difference
between trial type 3 and 4, however, the faster ones should be given a
stronger weight.

Our questions are:

(1) Is parametric modulation the right way to include such behav. measures?
(2) If yes, if we assume that faster reactions (lower reactions times) are
associated with higher BOLD response,
how is this relationship correctly encoded in the first level DesMtx? (It
seems that direct use of reaction times flips the relationship...)
(3) If we are still interested in T-contrasts (rather than just F-contrast
based differences), how are these contrasts interrogated? Do we need
another Design matrix that lacks the binary (unmodulated) regressor?

Thank you very much for any advice here,

Max Planck Institute of Psychiatry
NMR Research Group
Kraepelinstr. 2-10
80804 Munich
Mail: [log in to unmask]
Phone: 0049-89-30622-413