Thank you Donald for your comments.
Actually I'm not the guy who asked the quesion, but got interested in the
topic and involved in the exchange. I am also puzzled by this
"task-independent" connectivity. Let's leave aside for a moment the
question whether we can convincibly isolate any signal totally independent
of a task (well maybe at least partially independent). The question remains
kind of brain operational mode we study looking at such connectivity? Are
these some processes going on at the background; unconscious, partially
conscious? Or are we studying some default functional organization of the
brain, which is steady and independent (or to some degree resistant) to
what we are doing at a particualr moment? Probably there is something of a
default oragnization which doesn't get fully washed away by a task
demanding attention to external stimuli? So maybe it's something worse
exploring, even with some degree of approximation only? What about this
approach with ICA, as described in:
http://cds.ismrm.org/ismrm-2000/PDF1/0241.pdf
Iwo
On Aug 29 2011, MCLAREN, Donald wrote:
>Peter,
>
>A couple of comments:
>
>(1) I'm not sure the F-test is what you want. What you actually want is
>Y-BX. This might be possible with an F-test of zeros(1,10) 1. This would
>remove the signal due to the first 10 covariates since the constant is the
>same across all time points.
>
>(2) I've always thought about this approach, but two things have always
>bothered me about applying resting state connectivity to task data:
>(a) there is an implicit assumption that the task regressor explains the
>task; however, there is a huge amount of variability in the HRF across
>trials (see Huettel et al. 2001);
>(b) there is a second implicit assumption that the connectivity and the
>variability of connectivity is independent of the task and the evoked
>response. To my knowledge, no one has demonstrated that this is a plausible
>assumption.
>(c) finally I haven't seen any papers describing how violations of these
>implicit assumptions effect the results and their interpretations.
>
> Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D.
> Postdoctoral Research Fellow, GRECC, Bedford VA Research Fellow,
> Department of Neurology, Massachusetts General Hospital and
>
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>On Sat, Aug 27, 2011 at 4:24 PM, Peter Michalsky <[log in to unmask]> wrote:
>
>> 2. what is the point of defining constant term as your eof, it's just a
>> scalar: mean signal for the whole session, so there is no variance there
>> at all?
>>
>> we are talking about the first-level here. Hence, the constant is
>> actually estimated for each time-point and therefore should give me what
>> I like to think of as spontaneous activity of a particular region after
>> controlling for everything else. Correlating the spontaneous activation
>> signal between two regions then gives me a measure of functional
>> connectivity.
>>
>
--
Iwo
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