Contrast Masking and Differential fMRI Brain Responses Relative to a Rest Condition
|L. Clark Johnson1, Todd L. Richards2, Andrew Poliakov3, Elizabeth H. Aylward2
1School of Nursing, University of Washington
2Department of Radiology, University of Washington
3Department of Biostructure, University of Washington
Analysis of an fMRI protocol often involves a comparison of the fMRI
brain signal between task A (e.g., zero-back working memory task) and
task B (e.g., 2-back working memory). When a third reference condition
such as fixation is included then comparisons between the main tasks
and this reference are also possible (e.g. 2-back versus fixation). In
this case, the fMRI signal from each voxel’s response with regard to
fixation can be classified into one of six possible response patterns
(1), three for A > B (both A and B > fixation; both A and B <
fixation; A > fixation > B) and three for B > A. |
The objective of this project was to develop a method to separately map
these six different fMRI brain response patterns in addition to the
standard task A versus task B comparisons. Understanding the
relationship between activation for each task in comparison to fixation
is especially important when evaluating response to intervention in
Methods: Analysis was carried out using FEAT (FMRI Expert
Analysis Tool) Version 5.1, part of FSL (FMRIB's Software Library,
www.fmrib.ox.ac.uk/fsl) (2) using a standard block design with a 2-back
paradigm using pictures of sea creatures (the subject presses a button
if the current picture is the same as the picture present two before)
and a 0-back task using pictures of whales ( subject presses the button
if the picture is a whale). The contrast masking feature of FEAT was
used to generate images that mapped voxels with specific response
patterns (e.g. six images corresponding to the six response patterns).
Clusters with voxels reflecting significant between-task differences
were also identified for each of the one-sided comparisons of 2-back
versus 0-back (e.g. A > B and B > A). Software was prepared to
code each significant cluster voxel with regard to the response pattern
from which it was derived. The results of all six comparisons were
rendered onto a 3-dimensional structural brain using a cross platform
Java/Java3D software toolkit for processing and visualizing brain
imaging data (3).
Results & Discussion: Figure 1 shows
the fMRI overlay of brain activation which results from the six
different contrast masking conditions. Each of the response patterns on
the left margin of Figure 1 represent between-task comparisons that
will be significant when A > B (e.g., back represents voxels A >
B and both A and B > fixation), and response patterns on the right
margin are significant when B > A. As can be seen in the Figure,
there were contiguous voxels that have different response patterns even
within the same cluster.
Conclusions: Contrast masking may
be a useful method to characterize the fMRI signal response patterns
that can exist in on-off block designs.
Contrast masking may be a useful method to characterize the fMRI
signal response patterns that can exist in on-off block designs.
References & Acknowledgements: 1) Gusnard DA and Raichle ME, Nature Reviews, 2,685-694, 2001.
2) Smith SM et al, Neuroimage, 23, S208-S219, 2004.
3) Moore EB, Poliakov AV, Brinkley JF, 2004, 1761, 2004