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Hi,

The "particular waveform" is the custom 1-entry waveform specified by  
the file.
Usually you do want to convolve this with the HRF as the values in  
the 1-entry
file often specify stimulus-related events.  One example of this is a  
single-event
stimulus where the 1's in the waveform represent single stimuli and  
the remaining
values are 0.  In this case you want to account for the haemodynamic  
delay and
dispersion by convolving with the HRF in order for this model to fit  
the data.

An example of when you do not want to use convolution is when the  
entries relate
to something other than stimuli - something that directly affects the  
signal,
without being subject to haemodynamic processes.  This is often the  
case for
artefacts such as those caused by motion, respiratory or cardiac  
processes.  If
you have recordings of these and you want to regress them out of the  
signal, then
you can use the custom 1-entry file but you should not convolve with  
the HRF.

I hope this makes things clearer.
All the best,
	Mark



On 20 Nov 2007, at 06:19, Soohyun Cho wrote:

> Hello FSL experts,
>
> Can I get some clarification about some explanations in the feat  
> manual?
>
> In the feat in detail user guide, it says:
>
> " For a single-event experiment with irregular timing for the  
> stimulations, a custom file can be used.
> With Custom (1 entry per volume), you specify a single value for  
> each timepoint.
> The custom file should be a raw text file, and should be a list of  
> numbers, separated by spaces or newlines, with one number for each  
> volume (after subtracting the number of deleted images).
> These numbers can either all be 0s and 1s, or can take a range of  
> values. The former case would be appropriate if the same stimulus  
> was applied at varying time points; the latter would be  
> appropriate, for example, if recorded subject responses are to be  
> inserted as an effect to be modelled.
> Note that it may or may not be appropriate to convolve this  
> particular waveform with an HRF - in the case of single-event, it is."
>
> 1. what is "this particular waveform" referring to in the last  
> sentence? (the latter? or both the former and the latter ?)
> 2. what does it mean by "in the case of single-event"  in the last  
> sentence?
> 3. can you give me some more examples of when it is appropriate to  
> convolve a covariate with an HRF and when it is not?
>
> thank you for your help in advance,
>
> -Soohyun.