Regarding 'task related motion' -
is it not the case, that one would usually convolve task induced activations with some hrf while task related motion would affect the images immedately? Of course, one might venture that not just one scan will be affected by motion and thus the delay introduced by the hrf (probably at the order of 1-2 TR) would be 'eaten up'...
Just a thought,
Helmut
----- Original Message -----
From: Russ Poldrack
Sent: Wednesday, May 04, 2005 2:15 PM
Subject: Re: Motion Correction
3) how should task-correlated motion be dealt with?
is some degree of correlation to be expected, given
that in most paradigms, subjects are required to make
some kind of overt response?
Task related motion should be dealt with in pretty much the same way as any
other confounding effect in the statistical model. If you don't model the
confound, then you accept that the significant differences you see could be
explained by the confound. If you do model the confound, then you risk any
real effects being explained away by the confound.
One can also try to deal with these issues at the level of design - in particular, if you know that there is going to be task-correlated motion, then you can use an event-related design to try to decouple the motion from the task-induced BOLD signal, taking into account the fact that the induced signal is delayed with respect to the effects of motion. People have used this approach, for example, to allow subjects to speak in the scanner.
cheers
russ
---
Russell A. Poldrack, Ph.d.
Assistant Professor
UCLA Department of Psychology
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