Hi Ella, sorry for the delay. The VRF gets reduced by a factor roughly equal to TR/dPSD, so 5 in your case. Whether this matters depends on how you eventually want to analyze the data. If you are going to assume a shape to the HRF (eg, using SPM or FSL), then it won't make much difference since the realized VRF will be 5 times your 4.5 which puts you in a good range. If you plan to analyze using an FIR (ie, don't assume a shape), then you might want to add some runs to boost the power.

doug


On 7/29/13 4:06 PM, Ella Hinton wrote:

Hi Doug
Thanks for your email. I gave your suggestion a try  - reduced the dPSD to 0.5 (as well as adjusted the PSDMAX, and no. of volumes to reflect the new TR - see full command line below).  Unfortunately the efficiency is way down (0.03) and the VRF you previously suggested should be between 20-40 is 4.5!  I now wonder if Optseq is trying to tell me this is now a bad design!!

Would be very grateful for any ideas for improvement?

optseq2 --ntp 240 --tr 2.5 --psdwin 0 20 0.5 --ev hf 3 50 --ev lf 3 50 --ev nf 3 50 --nkeep 3 --o test14 --nsearch 10000 --focb 100

many thanks
Ella

Date: Sun, 28 Jul 2013 23:33:07 -0400
From: greve@nmr.mgh.harvard.edu
To: freesurfer@nmr.mgh.harvard.edu
Subject: Re: [Freesurfer] dPSD and ev duration

Hi Ella, it has an internal clock that events have to be syncrhonized to. Try setting the dPSD to 0.5
doug



On 7/28/13 4:05 PM, Ella Hinton wrote:
Hi

Quick question - why must the duration of the EVs be an integer multiple of the dPSD, which is also tied to the TR? I have event duration of 3 seconds, but would like to reduce my TR from 3 down to 2.5, but I can't seem to input the correct numbers for optseq to help me.

thanks in advance
Ella 


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