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ABSTRACT Colin McCulloch, Assistant Professor of Biostatistics
These are good times for statisticians in functional image analysis!
The data is messy, the application is trendy, and the results are
important. In this seminar, I will talk about the beginning of my
journey into functional imaging. In one type of fMRI experiment,
called a "box-car" design, a subject is scanned for several minutes
during which time, half the time she performs some task (eg. looks at
a black and white checkerboard on a monitor) and the other half she
performs some other task (eg. looks at a black screen). The tasks are
generally alternated for the duration of the experiment. Throughout
the experiment high-resolution, but noisy, images representing blood
flow in the brain are acquired and the goal is to quantify differences
in these images taken under the stimulus state (checkerboard screen)
and the control (black screen) state. Current analysis techniques
generally employ a simple t-test comparing stimulus image intensities
versus control intensities at every pixel in the image. However,
correcting for multiple comparisons is difficult since each image
contains around 100,000 pixels. Bonferroni won't help us here! The
state of the art in fMRI analysis is to manually choose an effect size
threshold that makes the effect images look "right"! Other
non-parametric analysis methods have been devised, but the effect size
threshold problem persists. I will discuss two Bayesian analyses I
have performed, one on a box-car experiment and the other on a
so-called single-trial experiment. Bayes seems to be the way to go
with these data for several reasons I will discuss, one of which is
that it effectively nullifies the multiple-comparison problem
tormenting the field.
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