Dear Aaron,
> Can someone please tell me, in an fMRI/boxcar analysis with multiple
> confounds and for a specific contrast, what EXACTLY gets plotted in a
> fitted/adjusted response (vs. time) plot? I thought this plot showed
> the estimate that results from the effects specified in the contrast,
> along with the data adjusted by subtracting the predictions of the
> confounds. I must have this wrong, because some of the estimated
> response plots that I have seen cannot possibly result from the
> specified effects alone.
You are correct that the fitted effects (specified by a contrast) are
adjusted for the remaining effects in the design matrix. In relation
to SPM96 the adjustment now considers anything apart from the contrast
of effects as confounds. By using F contrasts you can now adjust for
different effects post-estimation. This removal may produce unlikely
shaped responses. To see the fitted repsonses, without removing
anything but session effects, select the F contrast for 'all effects of
interest'.
If this still looks different from a convolved box-car then your
temporal filtering in probably too severe. The fitted and adjusted
data are for filtered data. Therefore the fitted responses are
expressed in terms of filtered regressors. These can have an
unexpected shape if too many low frequency components have been
removed.
Although this may be confusing at first we thought it was important to
show the fitted effects one was actually making an inference about.
Furthermore the use of this plot facility, in conjunction with
different contrasts, allows you to dissect different components of the
response and characterize them graphically.
With best wishes - karl
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