Hi All,
I agree with all that Tim wrote. Rik Henson has published a few papers on optimizing event related designs - you can find them here http://www.mrc-cbu.cam.ac.uk/~rh01/refs.html#_meth. Looking through them might help.
Tony
-----Original Message----- From: Tim Andrews [mailto:t.andrews@psych.york.ac.uk] Sent: 15 November 2007 09:58 To: ynic-users@ynic.york.ac.uk Subject: Re: Event-Related optimum schedule
Hi Laura,
Rapid event-related fMRI has many advantages for cognitive neuroscience research. However, the main problem with this technique is that the signal is very small compared to a block design. One way to increase the signal-to-noise ratio is to increase the number of trials. This typically means using a short ISI (otherwise your subjects could be in the scanner for a long time!). It is also good to vary the ISI to avoid expectation effects (i.e the subject predicting when the next event is likely to occur). However, the problem with a short ISI is that the response to one stimulus will likely overlap with the response to the next. This is not a problem if the BOLD response is linear (i.e. the response to two successive stimuli is the same as adding the response to two independent stimuli with an appropriate temporal offset). However, a number of studies have found that there are significant non-linearities when the ISI is less than ~5sec (eg Dale and Buckner, 1997; Huetell and McCarthy, 2000). So, varying ISI can have positive and negative effects on the BOLD signal. I haven't used the programs that Claire and Silvia are using, but I assume they are trying to find an optimum balance between these effects.
Users - please feel free to correct or comment!
Tim
Laura Lee wrote:
Hi MRI-support,
I'm struggling along trying to work out how to create a 'stochastic' event-related design for fMRI. Claire Moody has passed on a program that searches for the optimum stimulus schedule (she & Silvia used it for their last project). I've read over all the bumpf but am still quite confused by all the new concepts. I think I know roughly what I want but then there are some parameters I am unsure about and don't really understand the implications of the settings. I'd be really grateful if you could give me a hand.
This is the programme I downloaded... http://surfer.nmr.mgh.harvard.edu/optseq And there's a pretty comprehensive help page here... http://surfer.nmr.mgh.harvard.edu/optseq/optseq2.help.txt
Thanks, Laura