[pymvpa] Ignore volumes in 4d file?

[Shane Hoversten]

Howdy -

I have a whole bunch of nifti volumes that I'm using as a dataset.  At
first I was specifying all the volumes as a list of filenames to
fmri_dataset(), but I started getting errors about having too many
files open.  So I created a 4d volume from the relevant 3d volumes,
and used that, and all seems to be working.

Now I'm increasing my dataset to use even more volumes, and to do a
number of MVPA analyses on various conditions within those volumes.
For instance, among our conditions are "conc A" and "conc B"  and "abs
A" and "abs B".  Till now, I've just been using targets that are a
superset of these conc/abs classes, and every volume in the 4d file
belongs to one of these target "superclasses."

But now I'd like to break out, say, "conc A" and "conc B" as targets,
which means that some of the volumes in the 4d file will no longer
correspond to either target.  I could, of course, create another 4d
file, made out of just the volumes pertaining to the "conc A" and
"conc B" conditions I'm interested in; but it would be easier if I
could just have one "master" 4d file, and specify my targets within
that master file, and just ignore the volumes for conditions I don't
care about.

In essence, I'd like to specify the targets as something like
['concA', 'concB', None, 'concA', None, 'concB'] and so on, where the
"None"s correspond to volumes that should be ignored.  Is there a way
to do this, or something like it?

Shane