I recently came across a web-page I should have committed to memory years ago, when I was first starting to get to grips with SPM analysis:

Matthew Brett’s Introduction to SPM Statistics

It’s a fantastically straightforward guide to how SPM analysis uses a regression model defined by your contrasts to¬†establish the voxels in your scanner images that have significant activations.

It doesn’t take too much understanding on top of what you get from this web-page to appreciate that when you’re specifying that you want onsets modeled as a haemodynamic response function (hrf), the software is simply building a timecourse by stacking hrfs on top of one another according to your design-defined onsets. ¬†It them fits the regression which is now defined by parameters resulting from the estimated hrf timecourse rather than task difficulty values from 1-5.

I’d say this should be required reading for all those getting to grips with SPM.

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