Hi Lili
Just to make sure we're on the same page...The microtime resolution is
the number of time bins SPM divides each TR into for calculating
regressor values, and is stored as SPM.T. The microtime onset is the
bin number, of however many you've specified in SPM.T, from which the
regressor value is chosen to be entered in the model. The microtime
onset is SPM.T0. So the default settings of SPM.T=16 and SPM.T0=1
mean that each TR is divided into 16 time bins, and the value of each
regressor in the first bin is entered in the model.
> The manual recommond not changing the defauft settings (microtime
> resolution: 16, microtime onset: 1) unless a long TR, while in the Face
> example (Page 194), these two parameters were set according to the sample
> parameters (24 for the former and 12 for the later) .
A common approach, especially if slice timing correction is
implemented, is to set the microtime resolution to the number of
slices, and the microtime onset to the middle slice. One reason to do
this is that the middle slice (in time) is never more than TR/2 away
from any other slice; if you set the microtime onset to the first
slice, it is nearly twice that (TR) away from the last slice.
However, I would have thought with a 2-3 second TR, this would not
make a lot of difference.
> I change these settings in my data analysis, using the default settings (16
> and 1) or use the parameter during data acquiring. This caused some
> differences in the final result (second level analysis). Then which
> parameter setting approach is more reasonable?
I would expect changing the microtime parameters to have some effect,
but probably not a large one. I think that either setting would be
reasonable. I tend to favor setting the microtime onset to the middle
slice, and don't know of any reason why this is not a good idea---but
perhaps there are are some. Changing the microtime resolution I think
would have less of an effect in this case than the onset, but I don't
know of any downside to having a larger number (corresponding to more
time bins) for this.
Hope this helps!
Jonathan
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