When using the parametric modulation option for a 1st level analysis of an
event-related design, our design matrix does not appear as we expected.
We are using an HRF + temporal derivative to model brain activation in
response to to 4 conditions with a parametric modulator (e.g., reaction
time). For each onset time within a condition, there is an associated
parametric modulator value. The resulting design matrix consisted of 4
columns per condition. Why are there 4 columns? I guess we might have
expected 3 columns per condition. Does the 4th column correspond to the
parametric modulation of the temporal derivative?