I'm trying to teach myself how to analyze event-related designs.
In the worked example of the famous faces data in the SPM manual, the
instructions indicate the SOTs (stimulus onset times) are contained in
the sots.mat file, such that SOTs for condition N1 are sot{1}, N2 are
sot{2}, F1 are sot{3} and F2 are sot{4}. When I load the sots.mat
file, I can see sot{1} contains numbers like 6.75, 15.75, 18.00,
27.00, 29.25 etc.
What do these numbers represent exactly?
I expected these numbers to represent the scan during which, or
immediately after, each stimulus was presented. How could a number
like 6.75 represent a scan number? Does the extra fraction represent
jitter (SOA) so that 0.75 means the scan was actually three quarters
over before the stimulus was presented? If so, why not just use the
next scan (7)?
Apologies beforehand if this is a naive question.
Rich.
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