At the main menu, hold the Left Analog-stick and press Black, White, Y.

Successfully complete the game in Serious mode, then start a new game. Mental will be fading in and out under Serious. In Mental mode, all the enemies will appear and disappear.

In the Great Pyramid stage, just before you enter the pyramid, you may have noticed a panel above the entry with alternating I's and O's. This is a timer counting down in binary. The timer starts at 1,023 and goes down from there.