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
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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
Office: Martiusstr. 4, Room 6
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