Dane,

Duration modulation is usually used for the situation where the duration varies substantially from trial to trial. I don't fully know the nature and detail of your experiment design, so I cannot directly offer concrete suggestions for handling your scenario. In what sense the trial duration is varying? When the duration varies, two aspects of the BOLD response are potentially affected: the length and the magnitude of the hemodynamic response. The response length is roughly linear with the stimulus duration, while the magnitude has a nonlinear relationship with the duration (initially increasing but gradually getting saturated). The several options available in 3dDeconvolve are quite confusing because each of them may have different underlying assumptions.

As for isolating the effects associated with one particular condition through removing all other modeled effects, I don't feel the 3dSynthesize+3dcalc method is compelling. As there is substantial amount of effects not properly modeled, the removal might be largely ineffective.

Just throw in an alternative idea here (some people call it beta series correlation):

1) model each trial separately;
2) formulate your correlation matrix based on the beta series across the trials of the same condition.

With this approach, you don't need to worry about the duration variability, and you obtain the correlation matrix directly from the modeled effects instead of jumping through the tanged hoops.

Gang



Edited 1 time(s). Last edit at 01/15/2020 06:00PM by Gang.