Dr. Perry,
>
> It is perhaps just worth expanding on what you mean by 'very brief'.
> Because the haemodynamic response is so slow, huge changes in the
> time-course of neuronal firing, such as from an impulse 5 ms wide to a
> box-car with a width of 1 second. or to two 5 ms wide pulses one second
> apart, would be expected to have very little effect on the time-course of
> the haemodynamic response, which will in each case be 'nearly equal to the
> impulse function'. The amplitude of course may vary.
I agree completely.
>
> Your particular criticism only applies if one is dealing with unknown
> patterns of neuronal firing over at least several seconds. Even these will
> often be adequately modelled by a box-car convolved with the haemodynamic
> response function, since subtleties of the form of the neuronal response
> may well be ironed out by the convolution. I can't imagine many
> experimental designs in which one wishes to acquire whole-brain images in
> which the selective averaging approach will still be advantageous,
It is rare, but it does happen. We have encountered regions with
unexpectedly long patterns of neuronal firing. And, as you say, these
hemodynamic responses are well modeled by the impulse function convolved
with a boxcar function. However, the experimenter must know the width
of the boxcar a priori. This requirement biases the results since the
only regions will be detected are those that satisfy this assumed width.
This consideration is not important for the vast majority of studies.
However, improved sampling obtained with basis functions is also
only a slight benefit. You have a choice when you do a study: do you
want a slight increase in sensitivity by using temporal basis functions
(I am assuming that the TR is sufficiently short, say < 2.5 seconds), or
do you want to be able to detect regions with unusual hemodynamic
responses (and get an estimate of the hemodynamic response at each
voxel). We have implemented both methods, so each investigator can
choose the method he prefers. There is very little published data to
guide this choice, so opinions tend to be very strongly held.
John
>
> Best wishes,
>
> Richard Perry.
>
> from: Dr Richard Perry BM BCh MA PhD MRCP(UK),
> 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]
> Pager: 04325 253 566.
>
>
>
--------------------------------------------------------------
John Ollinger
Washington University
Neuro-imaging Laboratory
Campus Box 8225
St. Louis, MO 63110
http://imaging.wustl.edu/Ollinger
On Fri, 5 Mar 1999, Richard Perry wrote:
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