Hi Mike,
It looks like you got some good responses already. I just wanted
to add my $0.02 to Debbie Hall's reply:
> Efficiency was gained since the haemodynamic response to the
> stimulus had effectively reached a plateau 10-14 s after stimulus onset
> thus maximising any differences between ON/OFF states. However this
> method tells you nothing about the time course of the response - for
> that you would need to use a sparse imaging protocol as used by Belin et
> al (99 neuroImage) for example.
The assumption that the HRF response is maximal after a 10-14s delay
embodies two sub-assumptions. One concerns the time course of the HRF
(maximal after 6s approx) and the other concerns the shape of the
elicited response. In primary sensory areas, such as the auditory cx,
a constant stimulus seems to produce a constant response. So in
Hall's study, if I understand correctly, a constant tone for 14s would
produce a maximal HRF response.
In a study using auditorily presented words at some fixed rate, this
second assumption is unlikely to be true. Because the stimuli is not
constant, there won't be a steady-state response in the auditory cx.
Similarly, outside of primary sensory cortices, there may not be
steady-state responses anyway, particularly not for words (see Price
et al., 1999 NeuroImage, 10, 36-44; note this has been replicated in a
second study under review at the moment). So you may have suboptimal
HRF responses when you acquire your signal, particularly in 'semantic'
or 'lexical' areas.
As always, I hope that more knowledgable SPMers will correct any
mistakes I made.
Cheers,
- Joe
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Joseph Devlin, PhD * email: [log in to unmask]
Center for Speech and Language * [log in to unmask]
Experimental Psychology Department *
University of Cambridge * Phone: 01223 766 454
Cambridge, CB2 3EB * Fax: 01223 766 452
England *
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