The 'u' key (like 'o') applies to the image window.
The image window must be "selected" and the
mouse pointer must be over the actual image
portion of the window.
Similarly, when the mouse pointer is over a
selected window, hitting F1 brings up a help
window describing many key actions.
Note that for the 'u' key to matter, the underlay
and overlay need to be different datasets.
Also, looking at any 'afni' window title bar, they
will (usually) start with [A], followed by 'u' or 'o',
depending on which is being displayed as the
underlay.
- rick
