i think it's control-S, at least that works for me. edit: actually i think you are trying to do control-option-S to reset, not just turn on and off. anyway, ^ means control but the full up arrow (can't reproduce it here) means shift.
but... after a while a lot of keyboard shortcuts stop working on OSX. shift-apple-R,G,B,0 to change the view to R,G,B or RGB often stop working. and sometimes control-S stops working as well.
i assume this is another Qt bug...