On 06/04/2013 09:02 AM, Laura Dekkers wrote:
>
> 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
>
> --
> Laura M.S. Dekkers, MSc
> Research assistant
> University of Amsterdam - Department of Developmental Psychology
> Weesperplein 4
> 1018 XA Amsterdam - The Netherlands
> E-mail: