Dear Andre
An interesting problem! My 1p's worth:
Separating psychological and haemodynamic refractoriness may be not be
possible if you predict 'dual-task' region/s that won't respond in any
single-task condition - then you have a perfect confound (given what you say
about limited usefulness of different SOA ranges).
But if you could create a single-task condition that would stimulate that
region you could use that to test the region's HRF nonlinearities and
potentially separate them from cognitive nonlinearities in the dual task
condition - over the same time periods (range of SOAs). Something like the
difference between T1 T2 with varying SOA, and T1 T1 with varying SOA - as
long as your process overlap model doesn't predict just the same kind of
bottleneck in both!
Alexa
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| -----Original Message-----
| From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
| Behalf Of Andre Szameitat
| Sent: 14 April 2010 13:31
| To: [log in to unmask]
| Subject: Re: [SPM] Can these non-linear effects be corrected?
|
| Dear Chris,
| thanks for your input.
|
| > I think if you properly design your experiment, ensuring a range of
| > SOA's, you can still estimate the contribution of each condition
| > (assuming linearity). See e.g. Miezin et al. 2000 NeuroImage, Fig. 1.
|
| Well, but that's exactly the point: Due to the short SOAs/ITIs, we
| can't assume linearity. Therefore, the analyses in Fig.1 of Miezin (who
| demonstrated nonlinearities as well) doesn't hold.
|
| > But, for SOAs below 2000ms linearity might not hold.
| > Maybe you could do something like in Ollinger et al 2001
| NeuroImage...
| > they separated components of trials.
| Thanks for these suggestions, but presently I can't see how their
| method can help. They basically show that they can disentangle two
| temporally overlapping responses, but that doesn't tackle the problem
| that there may be two confounded effects, i.e. changes in cogntive
| demands and physiological non-linearities. In their case, there were
| only the non-linearities, but no postulated changes in cognitive demands
| (they investigated V1).
|
| > If you're interested in varying the SOA and looking at its effect on
|
| > activity, perhaps you could do some kind of parametric modulation?
| > Or maybe you can set up a factorial design, 2 x 2 (task A, task B;
| short
| > SOA, long SOA)...
| All this doesn't "de-confound" the two factors. The only thing I
| thought of inbetween was to use indeed a parametric manipulation
| spanning a wider time range, e.g. 0 - 6 seconds. The idea is that
| demands on cognitive processes (dual-task coordination) should start to
| increase only with SOAs < 2s, while non-linear effect may already be
| present with SOAs around 5-6s. But I'm not sure whether two slightly
| different timecourses of the effects are sufficient to disentangle them.
| An additional problem is that we don't know for sure the *exact* change
| in cognitive demands with decreasing SOA, so we can't set up a perfect
| regressor modelling them.
|
| best wishes,
| Andre
|
|
|
| >
| > Andre Szameitat wrote:
| >> Dear SPMers,
| >> I've sent this question before but didn't receive an answer. However
| as
| >> this was directly before the Easter holidays so that not all may
| have
| >> read it and this question is quite important for me, I'd like to
| post it
| >> again:
| >>
| >> I have a question about a very specific situation, in which
| non-linear
| >> effects may occur (due to the refractoriness of the hemodynamic
| >> response, i.e. the second of two successive identical stimuli evokes
| a
| >> smaller and delayed BOLD response). However, in my view they cannot
| be
| >> corrected - but I would be happy if you prove me wrong :-)
| >>
| >> The situation is a dual-task situation: Suppose you have two tasks
| A
| >> and B, which are performed with varying temporal distance (stimulus
| >> onset asynchrony, SOA). In cognitive psychology, it has been argued
| that
| >> the shorter the SOA, the higher the demands on executive functions,
| e.g.
| >> related to the coordination of overlapping task processing. Thus, as
| a
| >> first effect, in a dual-task related brain area X we would expect
| >> increasing levels of activity with decreasing SOA.
| >>
| >> However, it is not uncommon that already the individual performance
| of
| >> each task activates such a dual-task related brain area as well.
| Thus,
| >> suppose task A performed alone activates area X, and task B
| performed
| >> alone activates area X as well. When we decrease the SOA to create
| a
| >> dual-task situation, then there should be non-linearities due to
| the
| >> hemodynamic refractoriness. This is particularly so because the
| time
| >> range of hemodynamic refractoriness and SOA manipulations in
| dual-task
| >> paradigms is identical (usuallly SOA varies between 0 and 1500/2000
| ms).
| >> Thus, as a second effect, in a dual-task related brain area X we
| would
| >> expect decreasing levels of activity with decreasing SOA.
| >>
| >> Taken together, it appears to me that there are two perfectly
| >> confounded effects: Psychological processes predict an increase of
| >> activity with decreasing SOA, while non-linear BOLD effects predict
| a
| >> decrease of activity with decreasing SOA. Stated more generally,
| both
| >> effects result in a parallel (negatively correlated) modulation of
| the
| >> BOLD response. Therefore, it seems to me that such non-linear BOLD
| >> effects cannot be corrected. For instance, an additional regressor
| like
| >> the one added if Volterra kernels in SPM are used should be fully
| >> colinear with a regressor encoding the SOA (the psychological
| >> manipulation).
| >>
| >> It would be great if I am missing something and there would be a
| method
| >> that these two effects can be disentangled. Thus, any input is
| greatly
| >> appreciated.
| >>
| >> Kind Regards,
| >> Andre
| >>
| --
|
| ______________________________
|
| Dr. Andre J. Szameitat
| Department Psychologie
| Neuro-Cognitive Psychology
| Ludwig-Maximilians Universität
| Leopoldstrasse 13
| 80802 München, Germany
| Tel. +49-(0)89-2180 6778
| Fax. +49-(0)89-2180 4866
| www.psy.uni-muenchen.de/ncp
|
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| ______________________________
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