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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