Hi Stephen,
>>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.
>
>
Well I agree with the low frequency as I said in the previous e-mail..
but for A and B separately I really don't 'see' the prblm. Why, taking
your example, estimates would be -1 and 1? Given that A and B are
totally separated within the session it's similar to run two separate
sessions (but without the pblm of two sessions), one with A and one with
B (except the transition A r B). The increasing of the signal along the
session is possible but I don't see really why it should be? is there
any paper showing that? or is that related to something else?
>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.
>
>
Well, of course, it was about psychological/physiological factors,
detection power is clearly superior.
Best
- cyril
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