Hi Jeff
>> A few questions about the use of time and dispersion derivatives:
>>
>> If one uses time and dispersion derivatives in a model, does this
>> mean that
>> error variance associated with the canonical HRF is "soaked up" in the
>> model? If this is the case, then why wouldn't these be included as a
>> matter
>> of routine when employing the standard HRF? I understand there are
>> waveforms that are devoid of some of the canonical HRF constraints, but
>> wouldn't using one of these mean that you are just incorporating into
>> your
>> model what would otherwise be error variance in the canonical HRF
>> situation?
>
>
To add on Alle comments, the reason not to include derivatives all the
time is that you almost always look at the hrf for activations and that
for random effect analyses, this does not change (in theory) the
results. Indeed, at the first level the statistical power is increased
by decreasing the error (as you add two regressors) but at the second
level, because you pick up only the hrf regressor and use the between
subject variance, results are similar (though you can use derivatives at
the second level)
Jesper Andersson wrote greate explanations on that .. should be in the
archives
Best
Cyril
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