15 Nov
2007
15 Nov
'07
10:11 a.m.
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(a)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
--
Dr Tim Andrews
Department of Psychology
University of York
York, YO10 5DD
UK
Tel: 44-1904-434356
Fax: 44-1904-433181
http://www-users.york.ac.uk/~ta505/
http://www.york.ac.uk/depts/psych/www/admissions/cns/
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