On Wed, 26 Aug 2009 17:54:57 +1000, Richard Morris
<[log in to unmask]> wrote:
>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)?
It's more precise to use the exact time rather than the nearest scan.
SPM does initial computations on a timeline that's much finer than the TR; I
think this is sometimes called "microtime." Of course, there's no data for most
of those times; after the predicted waveforms are computed,
they're "downsampled" to the actual time intervals collected.
>
>Apologies beforehand if this is a naive question.
>
>Rich.
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