Hello Richard and Geraint,
Thanks for your excellent suggestions. As I'm
relatively new to SPM I wonder if you would mind clearing up
a few things up for me?
The data were acquired with a block design, 15 seconds OFF,
15 seconds on, etc, etc. Did the method Richard proposed require
some re-ordering of the data? Basically I'm not too sure about
how to specify the required model.
Also, considering modification of BOLD signal due to scanner
'variation' - does SPM detrend the time course data? If so
wouldn't a time dependent reduction in BOLD signal be physiological
rather than artifactual?
Thanks again for your help.
Yours,
Jon.
_____________________________________________________
Jonathan Brooks Ph.D. (Research Fellow)
Magnetic Resonance and Image Analysis Research Centre
University of Liverpool, Pembroke Place, L69 3BX, UK
tel: +44 151 794 5629 fax: +44 151 794 5635
On Wed, 26 Jul 2000, Richard Perry wrote:
>Dear Geraint (and Jon),
>
>>Richard also said:
>>
>>> One could imagine a condition by time
>>> interaction occurring non-specifically, perhaps even because of the
>>> physics of the scanner.
>>
>>Although I agree with Richard's general points, I'm not sure about this
>>specific point. Overall fMRI signal in all conditions may wander over time
>>for a variety of experimentally uninteresting reasons. For example, low
>>frequency drifts in signal are common to all conditions and usually removed.
>>But relative differences in signal between two conditions that change over
>>time ('condition-by-time interactions') have by their nature
>>condition-specific causes. I agree these may be trivial or uninteresting,
>>but in Richard's example I think would have to be some condition-specific
>>scanner physics that affected the active condition but not the rest
>>condition (or vice-versa) in a time-dependent way.
>
>I guess that, to give an over-simplistic example, I was imagining a
>case in which the scanner drifts into a state in which it is much
>less sensitive to BOLD contrast. During the first few scans there is
>a large differential response. As the session continues, the
>sensitivity to BOLD decreases, and therefore the differential
>response decreases. This would appear as a condition-by-time
>interaction (possibly even without any 'main effect' of time), but
>one could not necessarily infer that the physiological response to
>the 'active task' compared to the 'baseline' changes over time, which
>is what one is usually interested in.
>
>If I wanted to provide evidence for a genuine time-dependent change,
>my own preference would be to look for a three way interaction. If
>one could demonstrate, for example, that in context A there is a
>differential response (task - baseline) which changes over time,
>whilst in context B there is a differential response which does not
>change over time, then I would feel happier that the change is at the
>neural level. However, an alternative would perhaps be to look for
>some way of 're-setting the physiology' so that the decline in
>differential signal can be repeated several times during a single
>session, making a scanner-dependent explanation unlikely.
>
>Best wishes
>
>Richard.
>--
>from: Dr Richard Perry,
>Clinical Research Fellow, Wellcome Department of Cognitive Neurology,
>Darwin Building, University College London, Gower Street, London WC1E
>6BT.
>Tel: 0171 504 2187; e mail: [log in to unmask]
>
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