Wen -
SPM does not provide a delay time estimate (the parameters in
spm_hrf are fixed - based on previous empirical studies - and
do not vary in order to fit data).
You could:
1. simply find the PST for the max of the fitted response
(which you can plot in SPM), assuming you used more basis functions
than simply the canonical HRF.
2. estimate the delay RELATIVE to the peak of the canonical
HRF by taking the ratio of temporal derivative to canonical HRF
parameter estimates (eg, Henson et al, Neuroimage, 2002), though this
only really works within the linear range of the Taylor expansion
appropriate for that derivative (ie +/-1s from the canonical in SPM).
3. use an iterative technique (eg fminsearch in matlab) to
explicitly estimate the delay parameters of a function like spm_hrf
(eg, Henson & Rugg, HBM01 abstract).
Whether haemodynamic latencies are really that informative with respect
to neural latencies is another question however....
Rik
Wen-Ching Liu wrote:
>
> Dear John and SPM users:
>
> I wonder that can SPM provide the measurement of delay time for
> estimated HRF.
>
> In spm_hrf, it has the default values for delay time. But it does not
> provide
> the delay time of actual response function. The plot of fitted response
> seems to be hard to estimate. Is there any numerical value which can be
> obtained
> for this?
>
> Thanks
>
> Wen
>
> --
> ==========================================================================
> Wen-Ching Liu,Ph.D. Functional Imaging Laboratory
> Assistant Professor of Radiology, UMDNJ-NJMS
> 150 Bergen Street Newark, NJ 07103
> 973-972-2281 (fax) 973-972-0836
> ==========================================================================
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DR RICHARD HENSON
Institute of Cognitive Neuroscience
& Wellcome Department of Imaging Neuroscience
University College London
17 Queen Square
London, WC1N 3AR
England
EMAIL: [log in to unmask]
URL: http://www.fil.ion.ucl.ac.uk/~rhenson
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