Russ,
I am guessing (hoping) that your query is addressed mainly at the
distinction between 'Average Fiducial Mapping (AFM)' vs.
'Multi-Fiducial Mapping (MFM)', which are the two prime approaches we
currently recommend for mapping to the PALS atlas. If so, I posted a
response to Mike Fox last week (Oct 26th) that addresses this question.
However, it was buried in with several other issues, so I have
excerpted it below.
There are other mapping options in Caret besides AFM and MFM, and other
issues besides what is covered below, so if this doesn't answer your
questions, fire away again.
David
On Oct 31, 2005, at 6:20 PM, Russ Poldrack wrote:
Hi CARETeers - I have a question regarding algorithms for surface
mapping (in this case, mapping group data to the PALS atlas). I can't
seem to find any particular guidance regarding the strengths and
weakness of the various surface mapping algorithms. Can one of you
provide some suggestions regarding algorithm choice, or point me to a
resource that describes this issue in more detail?
cheers
russ
From Mike Fox:
The question I had concerned the validity of mapping to 12 individuals,
then averaging those results, as compared to mapping just once to the
average anatomy of the 12 individuals (a new function of caret). The
two do not always give the same result, and I was wondering if you felt
one was superior to the other and why. I know that volume space atlas
registration has adopted the second approach (ie data is warped to a
single atlas which is the average of multiple subjects anatomy), but
this does not necessarily make it superior.
DVE response:
For starters, it's useful to review what I said about this topic in the
Discussion of the PALS paper:
In order to interpret the results of MFM, it is important to consider
several underlying assumptions. Without access to the individual
structural and fMRI data in any given study, it is impossible to work
backwards from volume-averaged group data to determine what the actual
pattern would be in any individual. Hence, the activations seen on any
of the individual PALS-B12 surfaces do not reflect a pattern in fact
attributable to any of the actual fMRI subjects. Nor do they
necessarily reflect the pattern that would have arisen in the 12
subjects whose structural data contributed to the atlas if they had
been tested using the same fMRI paradigm. MFM does provide an objective
strategy for estimating both the region of most likely activation and a
plausible upper bound on the total extent of activation. This
constitutes an important advance over the common current practice of
mapping volume-averaged results onto a single-subject atlas.
In many situations, it is appropriate to map group-average data using
both AFM and MFM. The two mapping methods yield similar but not
identical spatial patterns and are inherently complementary. AFM is
conceptually simpler and allows readout of values at each surface node
that correspond to a particular voxel value. MFM provides a more
objective assessment of the most likely spatial distribution on the
atlas surface.
-----
In short, I contend that MFM is a superior way to estimate the most
likely spatial location of regions likely to have been modulated in any
given paradigm. AFM can give significant biases in spatial
localization, depending on the nature and location of the data.
However, a price is paid in terms of relating the surface node values
in an MFM map to the voxel values in the volume. In some situations
that's pretty important, but in others it may be largely irrelevant.
These issues are truly complex, as they are intimately linked to the
nature of structural and functional variability and what is really
meant by corresponding locations in different individual hemispheres.
I hope this is helpful. If you have further comments, questions, or
discussion points, let me know.
David
