hi Gang

I am now a bit confused. The help for 3dClustSim states:

-2sided = Normally, 3dClustSim does '1-sided' testing. The random
dataset of Gaussian noise-only values is generated, and then
it is thresholded on the positive side so that the N(0,1)
upper tail probability is pthr. If you use this option, then
the thresholding is instead done in a 2-sided way: positive
AND negative values will be clustered together, with the
thresholding chosen to have a 2-sided tail probability given
by pthr.
* This option is for experimentation, not for serious use

I take this to mean that 3dClustSim by default only looks for clusters within the smoothed noise whose values are all positive--i.e., it does not make 'clusters' composed of both positive and negative suprathreshold voxels.

Am I understanding this correctly? If so, why would the default change?

Any clarification you could give would really be appreciated.

thanks

James