Hello Jodene,
There is no tool in AFNI for doing automated gray/white segmentation. The only thing that comes close is the plugin called Gyrus Finder written by Rick Reynolds a long time ago for a different purpose. For more useful comments, we'll have to wait for Rick's return from: [
www.ecophotoexplorers.com]
I know some have used FSL's SIENA, you might want to give it a try, FSL's data can be read into AFNI for further analysis.
cheers,
-z