Dear All,
I'm new to fMRI, and have a couple of basic questions with regards to a
block design study we are planning to do, and would be grateful for any
advice.
Essentially, we are interested in the effects of encoding instructions on
neural activations during processing of certain images.
We would like to compare the effects of free viewing, and 2 types of
instructed encoding, on images that vary along 2 dimensions (valence and
complexity).
The instructions are kind of complicated, and so I wondered if it would be
possible to directly compare activations between different runs of several
blocks. So, we would have a run of ~ 4 mins of free viewing, with 30s
blocks of two types of pictures (say a, b), so we would have a run
a1b1a1b1a1b1 etc. Then we would have a run of 1 type of instructed on the
same images a2b2a2b2 etc, and then a run of a different type of instructed
encoding a3b3 etc. We then have a final block of instructed encoding, but
only for neutral (a) images, so this would look like a4a4a4a4 etc..
My question is this. If we counterbalance the order of the runs, is it
valid in SPM to directly compare levels of activation between runs? i.e.
compare all blocks in a run of instructed encoding, versus all blocks in a
run of free viewing? Or, would one have to alternate 30 secs of differing
encoding instructions within the same run?
The reason we would like to do this is that the instructions are quite
complex and we don't want too much "switching" going on within a run.
However, if we can't directly compare across different runs, is there a
disadvantage in lengthening task blocks to say 60 secs, rather than 30 in
terms of power? Alternatively, should we interleave 30 sec blocks of task
with 30 sec rest periods, within the same run, to allow us to present
instructions on the screen and to make it easier for subjects to task-switch?
With thanks in advance,
Andrew
_____________________________________
Andrew Lawrence PhD,
MRC Cognition and Brain Sciences Unit,
15 Chaucer Rd,
Cambridge CB2 2EF,
UK
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