On Tue, 9 Aug 2005 10:07:29 +0300, Cyril Pernet
<[log in to unmask]> wrote:
>Hi Sherif,
>
>>Hello,
>>A few years ago I came across the following design:
>>ArArArArBrBrBrB
>>
>>with r being a 'baseline' condition while A and B were 'neutral'
>>and 'emotional' conditions, respectively (r=15 sec, A and B = 30 sec
>>each). The rationale of the authors was that they wanted to avoid having
>>condition A contaminated by condition B (assuming it could happen if a
>>block or blocks of B were to be placed before a block of A during a given
>>run).
>>
>>
>I don't see anything wrong to separate A and B .. the prblm of blocked
>designs is always the same (assuming similar effect along the blocks)
>whether A & B are intermixed or not
I'd have to disagree with that. It's clear there's a significant low-
frequency component in the A-B contrast in the given design. The simplest
way to see that is to consider the case of a linearly increasing signal.
For simplicity, suppose we've already "regressed out" the mean, so the
signal starts out at (say) -1 and ends up at +1. In that case, clearly
the estimated value for the parameter corresponding to "r" is zero, to "B"
is positive, and to "A" is negative. So A-B shows up negative, and
doesn't appear to be negligible.
More precise numbers in a toy model can be obtained by inputting a simple
design into SPM2 and looking at the plot of power vs frequency for each
condition.
This doesn't mean that the design is illegitimate for certain, but rather
that there is an issue of statistical efficiency here. If the efficiency
is low enough, one might conclude that the design shouldn't be used.
Also, I'm not sure what you mean about the problem of blocked designs.
For detecting a signal (though of course not actual estimation of the
shape of the response), block designs are superior to event related
designs. Their disadvantage is that psychological considerations may
disallow them.
>
>>Is there anything wrong with this design (e.g. is it susceptible to low
>>frequency noise? Is it sub-optimal for some other reason?).
>>
>The frequency of the activation blocks might be low although above the
>usual 0.01Hz cutoff. You can probably improve the design with shorter
>blocks (but more blocks) and maybe different length blocks (e.g. 13 16
>19s) for activation as well as rest. Have a look at Rik's 'design
>efficiency' webpage, it is very very useful!
>http://www.mrc-cbu.cam.ac.uk/Imaging/Common/fMRI-efficiency.shtml
>
>Best
> - Cyril
>
>
>
>
>--
>Pernet Cyril
>Neuropsychologist, PhD
>INSERM U455 Pavillon Riser
>CHU Purpan 31059 Toulouse
>France
>Tel: +33(0)561779503
>Fax: +33(0)561499524
>http://membres.lycos.fr/wamcyril/
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