Hey Jonathan and Donald,
thanks for your answers.
Ok, so the inclusion of the HRF model was totally unnecessary, that was
a mistake on my part.
The attached image may help in giving you an idea of what I am trying to
do. It depicts the time course of a stimulus and the theoretical evoked
hemodynamic response. Below these curves are the non-overlapping FIR
regressors. The reason I am looking at only 1 bin is because I want to
get information from the dynamic response to the stimulus. I want to
know which areas show activation first, compared to others. I thought
the FIR method is a good approach, because it helps me model the HRF
sequentially in time rather well.
There is also the other approach to use time shifted stimulus functions
convolved with the HRF. This approach might work, but it makes a big
assumption about the HRF across different areas of the brain.
Regarding the window length, I have an ISI of 10 s (for a total of 20
presentations), which is half of what you suggested. I have a bin for
every scan (TR = 0.514 s) and I am still deciding on how many bins to
use. I should probably stick to 19 bins, otherwise I get into the next
stimulus response. I do understand that I have a lot less power by using
only 1 bin, but I cannot use an F-test over all bins, since this will
give me only one image over all time points.
If any of this is way off or makes no sense, then please correct me.
Regards,
Glad
On 17.07.2012 20:45, Jonathan Peelle wrote:
> Dear Glad,
>
> I'm not quite sure of how you combined the HRF model and FIR models,
> or what the reasoning is behind this - but I'm happy to comment on the
> FIR parts below. Sorry if I've missed something critical here.
>
> For an FIR model, the window length sets the total time you are
> modeling, and the order indicates how many basis functions to use. So
> in your case, let's say you leave the window length at 32 seconds for
> all of your models. Using 1 vs. 14 vs. 26 regressors to model the
> response will result in different time-periods for these regressors -
> i.e. regressor #6 in the model with 14 time bins will not be
> equivalent to regressor #6 in the model with 26 regressors. I'm also
> not sure how you set up your model so that a single FIR regressor
> would be equivalent to #6 in the other models - but obviously, this
> would affect your results as well.
>
> It's hard to say what the "optimal" design is - it depends on your
> data and your question! Testing a single bin for an FIR model is not
> really making use of the full power of the model - using something
> like an F test over all of the columns will probably be more
> sensitive. Though, these can be trickier to interpret, which is why
> many people prefer using an informed basis set (such as the canonical
> HRF). For an FIR model, you probably want to span at least 20 seconds,
> and have a bin at least every 2 seconds I would think.
>
> Hope this helps!
>
> Best regards,
>
> Jonathan
>
--
Glad MIHAI, M.Sc. Biomedical Physics
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