[pymvpa] Hemodynamic lag shift

[Yaroslav Halchenko]

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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