Pressing CTRL opens the menu, which contains the functions of the Ego Terminal.
After acquiring the Ego Terminal (and when you are not in an area with purple gas), you’ll notice that the ‘equip’ and ‘ego’ menus are now selectable. In areas with a save point you will now also be able to adjust the difficulty in the ‘settings’ menu.
(In a later version of the game the menus will look different after acquiring the Ego Terminal to make this clearer.)