I am learning to use SPM 99 to generate experiments. It seems that the
onset times for the various stimuli are contained in the matrix
Sess{1}.ons{n} for the nth event type.
I've noticed with several different designs that these onset times are
0.125 seconds later than I would expect: for example, the beginning of the
first epoch for a block design -- Sess{1}.ons{1}(1) -- is 24.125 sec when
I specify 12 scans (at 2 sec TR) as the time to first trial. This also
occurs for event-related designs.
I can certainly subtract an eighth of a second and use the results I'm
getting, but I'm curious to know whether I'm missing something.
Thank you.
Baxter Rogers
[log in to unmask]
fMRI Group
Medical Physics Department
University of Wisconsin - Madison
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