Dear SPMlist,
the Background:
In our experiment we want to find neural correlates of pain perception.
For each participant, we use a different physical temperature such that pain
ratings are equal across subjects
Details:
4 Conditions (Stim/Rest with/without Pain) , blocked Design, 8 Blocks per
condition, 25 seconds per block, TR=2.5, total length ~ 18min
Between 2 subsequent blocks there is a 'blank interval' during which the
temperature is ramped up or ramped down.
Structure:
... -blank interval(ramp_up) - Block - blank interval(ramp_down) - Block - ...
the Problem:
the thermal applicator has a constant ramping rate. Since destination
temperatures are variable across subjects, the time needed for ramping
up/ramping down during the 'blank interval' is variable as well. This means
the duration of the paradigm may vary by, say, 1-5 minutes across subjects,
and so will the Design Matrix.
the Question:
Is there a problem, when these Design Matrices, that differ in their number
of rows for each individual, are entered into a group analysis and are there
any logical pitfalls to consider in this case?
Note that the matrices only differ in the duration of the 'blank interval'
between two subsequent blocks - this time interval needs to be variable,
since the destination temperature is different for each participant.
Grateful for any advice,
andi
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