[pymvpa] Hemodynamic lag shift

[Vadim Axel]

Thanks for your reply.

My data is functional localizer of the FFA (blocks of faces vs. blocks of
objects). To avoid contamination I have 15 sec fixation between blocks. The
task is 1-back. So, nothing special (more or less like your experiment :)

1. In preprocessing I run slice time correction so it should be a problem.
3. Experiments session start is triggered by the scanner and the session
ends exactly with scan end.

Your proposal of ER analysis is interesting. I will do it. Actually, I can
run a stadard GLM for each TR of my block as though it was event related
design and then make t-cotntrasts for each event. However, for first TRs I
may get better results because they are not contaminated by previous signal
(remember, I have a fixation beforehand). Maybe this is also the reason for
my better prediction issue? BTW, what happens when I analayze a block desing
data as ER. I have no optseq so model fit should decrease. Correct?

Vadim


On Sun, May 24, 2009 at 6:03 AM, Yaroslav Halchenko
<debian at onerussian.com>wrote:

> Well -- it is indeed an interesting effect ;) various factors could
> contribute to it and it is hard to comment without specifics of the
> experiment/task/acquisition
>
> 1. volume is not an instantaneous measure, so it might be that the slice
> of interest is actually delayed already by a TR*(1-1/number_of_slices)
>
> 2. who knows -- pre-dip effect exists and some people claim that it has
> better spatial locality than full-blown response
>
> 3. if trial is not magnet-pulse triggered, it is easy to slowly
> accumulate some lag by the end of the series
>
> etc
>
> the best way to see what is happening imho is to do ER-like analysis by
> taking multiple volumes after each onset and then looking at the
> sensitivities for interesting voxels across time; and also to look at
> per-stimuli plots --- where is the most difference between responses to
> two conditions. Noone should fully believe into a static homogeneous HRF
> across all voxels/conditions.
>
> Michael, should we put your face/shoe pictures online -- they might
> benefit some people ;)
>
> On Sat, 23 May 2009, Vadim Axel wrote:
>
> >    Hi,
> >    I am getting consistently better prediction when I don't make
> >    haemodynamic lag shift. Have you ever experienced something similar? I
> >    would expect the opposite results. I have closely examined the code
> and
> >    there are no bugs there.
> >    Thanks,
> >    Vadim
> --
>                                  .-.
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> Yaroslav Halchenko              /(   )\               ICQ#: 60653192
>                   Linux User    ^^-^^    [175555]
>
>
>
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