Selection ToggleKeyCreator \ General-Interface \ Selection \ Selection-Toggle
The Selection Toggle control is located in the Tools>Options>Selection tab. This control will allow you to select a key, which will cause selection toggling. Reverse selection toggling can be achieved by depressing the SHIFT key.
NOTE: The default selection toggle is now the Spacebar, not the Tab key as it was in previous versions. The Tab key can be made the default, however, through the Tools>Options>Selection tab.