Hello,
Just a though on a fascinating discussion.
I agree that, ultimately, one needs to express task-specific functional
neuroimaging differences in terms of some other measure. However, the
absence of a behavioural measure in the presence of a functional imaging
activation does nothing, in itself, to abnegate the validity of the imaging
observation. Rather it feeds back to our behavioural measurements and
encourages us to explore the behavioural outcomes of our tasks more
thoroughly.
Consider the developing literature (itself rooted in ERP work) showing
that, if a subject learns a list of words under apparently identical
behavioural conditions, and then if subsequent behavioural memory retrieval
measures are used to categorise these words according to whether or not
they were subsequently recalled, then we can separate those words that were
successfully encoded from those that were not and identify a regional BOLD
response that characterises this difference. Here we have a behavioural
difference that is only obtained after scanning has occurred and the
challenge is to find a behavioural measure that is measurable during the
scanning and differentiates abetween successfully and unsuccessfully
encoded words as sensitively as does our imaging measure.
It seems to me that behavoiural measures may be no less subject to error
and insensitivity than imaging measures. Just as observing that an imaging
study fails to distinguish between two conditions that are clearly
separable at the behavioural level cast doubt upon our imaging findings, so
is the reverse true.
We must keep the faith and believe our imaging data.
Very best wishes
Paul Fletcher
At 08:51 31/05/01 -0400, you wrote:
>Dear Russ:
>
>I am sure I am being naive, but is it not the case that this obviates the
whole
>idea of functional, in fMRI. In the absence of any reliable behavioral
>indicators, as well as a control condition, how can we be sure that any
>activation
>is the results of that particular function. I see many papers in the
field that
>do this
>so I would really appreciate it if someone explained the logic of looking at
>activation
>across conditions when there are no behavioral differences across
conditions. It
>seems
>to me that in the absence of some reliable external behavioral indicator
we have
>circular
>argument for the role of that particular function in activation of a region.
>
>thank you in advance,
>
>Nader
>
>Russ Poldrack wrote:
>
>> I would think that it is quite reasonable to analyze the imaging data
across
>> conditions that don't show any differences in RT or accuracy,
particularly if
>> there are hypotheses about differential activation across the conditions in
>> question. For one, it could be the case that there is some cognitive
process
>> that varies across the conditions but the behavioral measure is not
sensitive
>> enough to find this difference whereas the imaging measure might be more
>> sensitive.
>>
>> cheers,
>> russ
>>
>> Greig de Zubicaray wrote:
>>
>> > Dear all,
>> >
>> > I have a thorny question motivated by a reviewer's comments on one of our
>> > fMRI papers.
>> >
>> > Briefly, we used a memory task with variable delay periods. However,
we did
>> > not find significant differences in terms of either accuracy or RT across
>> > the delays. Therefore, we did not look for an effect of delay in the fMRI
>> > data. The reviewer is requesting that we revise the paper to include this
>> > analysis.
>> >
>> > Does anyone know of any references addressing whether it's "good form" to
>> > perform analyses of neuroimaging data in the absence of a significant
>> > behavioural effect? Any views on this issue?
>> >
>> > Any help would be appreciated.
>> >
>> > Regards,
>> >
>> > Greig
>> >
>> > --
>> > Dr Greig de Zubicaray
>> > Centre for Magnetic Resonance
>> > The University of Queensland
>> > Brisbane, QLD 4072
>> > AUSTRALIA
>> >
>> > Tel: +61 (0) 7 3365 4250 [direct]
>> > +61 (0) 7 3365 4100 [CMR]
>> > Fax: +61 (0) 7 3365 3833
>>
>> --
>> Russell A. Poldrack, Ph. D.
>> MGH-NMR Center
>> Building 149, 13th St.
>> Charlestown, MA 02129
>>
>> Phone: 617-726-4060
>> FAX: 617-726-7422
>> Email: [log in to unmask]
>> Web Page: http://www.nmr.mgh.harvard.edu/~poldrack
>
-----------------------------------------------------------------
Paul Fletcher,
Box 189,
Research Department of Psychiatry,
University of Cambridge,
Addenbrooke's Hospital,
Hills Road,
Cambridge,
UK
CB2 2QQ
Tel 01223 336 988
Fax 01223 336 581
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