Should inter-stimuli-interval events be introduced into fMRI first-level GLM?

, , ,
1

Hi everyone,

Our group conducted a event-related fMRI experiment. A little question in first-level GLM construction confuses us these days:

Each trial in our experiment contained a cue screen (2 sec.), several decision points (5 sec.) and several inter-stimuli-interval (ISI, 2 to 5 sec. randomly) between decision points. The ISI was a white cross presented on a black background. Should we include the timeseries of the ISI in the first-level GLM?

We are pleasure to hear your advice. Thanks in advance!

2

Hi,
I would not model the ISI, as it should correspond to the implicit baseline of the experiment. Including it would might degrade the conditioning of the design matrix.
HTH,
Bertrand

2 Likes
3

Hi Bertrand,

Thanks for your reply. The idea in FSL course - first-level analysis also recommends not to model the null events in the GLM.

Could I ask a follow-up question if you don’t mind? We are interested in the effect of decision points in our experiment, not the cue screen. Should I model the timeseries of cue screen in the first-level GLM?

Thanks,
Qunjun

4

Yes, i would model the cue, to avoid the cue effect being confounded with the decision condition.

1 Like
5

Got it, thanks for all your help : )