Aperture: during Import, keys to check/uncheck an image? In Aperture 3, at the beginning of the Import process, one can select/check or unselect/uncheck images one by one.

But it requires the mouse.

In the Viewer view, during that selection process, I can go to the next image easily using the keyboard, with the -> key, but what is the way to uncheck an image using the keyboard? 

(ie without having to click on the checkbox with the mouse)

Edit

Since I'm here, and in order not to create another question:

is there a way to have the Undo to work only on the current image, being viewed in preview?

(the default for multiple undoes, is to perform the undoes even on previously edited images (not shown during the undo) - very confusing/dangerous)
What can you do is to select the photos you want to import by:


click and dragging
cmd + click to add one photo
shift + click to select the range from the current selected.


Then click on any photo's checkbox, and all checkboxes will be checked as well.