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Abstract:
The n-Back task, in which verbal working memory (WM) load is
varied parametrically, has been used to examine the neural
substrates of verbal WM (Braver et al., 1996, Jonides, et al.,
1996, Cohen et al., 1997). By examining regional activation x
memory load interactions, much has been learned about probable
functions of those regions. For example, monotonic increases in
activation with memory load have been observed in areas
hypothesized to subserve storage operations. While such empirical
work suggests the function of observed verbal WM areas, the
evidence for those putative functions is indirect. Structural
equation modeling, a theory-driven approach to modeling
interrelations of variables, has typically been used to model data
at the level of groups of subjects. Here, fMRI provided multiple
observations of each subject's activation during performance of the
n-Back task, allowing each subject's data to be modeled
individually. A theoretical model was constructed, using latent
variables to represent neural processing units. For example,
hypothesized language areas loaded together, forming a "rehearsal"
construct. This model's plausibility, which makes explicit the role
of regional activation, was examined with data from four individual
subjects. This analytical procedure appears promising because the
large number of observations available in fMRI provides substantial
power to test theoretically interesting models of the
interrelations of neural regions.
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