Dear Dr Greve,
Together with Dr Hilde Huizenga I am implementing an fMRI study in which we would like to optimize the design by using Optseq2. We have a few questions. Could you please help us out? Our task consists of 3 runs of 72 trails each, in which participants are presented with a fixation cross that remains on the screen for a fixed duration of 1 sec, followed by one out of nine different stimuli that remains on the screen for a fixed duration of 4 sec. This gives a scan duration of 360 sec. We would then like to add 25% jitter, which results in a scan duration of 450 sec, and as TR = 2.0, this results in 225 time points.
However, Optseq yields an error if ntp is set to 225, ev duration set to 4.0, psdwin dPSD to 1.0 , tnullmin to 1.0 and tnullmax to 6.0 ERROR: could not enforce tNullMax=6 (ntries=100000) You will need to reduce the number of time points or increase the number of presentations.
However, reducing the number of time points makes Optseq to only insert null events of 1.0 or rarely 2.0 sec without much variation. Optimization with ntp set to 225, ev duration set to 5.0 (=fixed duration of fix + stimulus), psdwin dPSD to 1.0 , tnullmin to 0.0 (1.0 does not work) and tnullmax to 6.0 does work. However, this renders Optseq to insert null events of a fixed duration op 1.0 sec, which is not what we want.
Could you please advise us in how to set these parameters?
Thank you in advance.
Best regards, Laura Dekkers