Thank you so much, Mark.
That helps a lot.
One last thing..
I'd like to make sure that I understand what it means by the
covariate being "related" to the stimulus.
What about covariates that are 'related' to the stimulus, but are not
of interest ?
For example, let's say I have a memory task using words. If I want to
regress out the effect of word concreteness (not of interest, but
related to the stimulus) for each trial, should I convolve this
regressor or not?
Also, if I want to regress out the response time for each trial (not
of interest, but related to the stimulus in that it reflects the
cognitive difficulty of the trial), should I convolve this covariate
or not?
Looking forward to your answer.
Thank you in advance.
Soohyun.
On Nov 19, 2007, at 11:53 PM, Mark Jenkinson wrote:
> 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.
>
>
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