Amplitude
Blurry Vision Mode
During gameplay, press R3, R3, R3, R3, L3, L3, L3, L3, R3.
Tunnel Vision Mode
During gameplay, press L3, L3, L3, R3, R3, R3, L3, R3, L3. Enter this code again for a different view.
Monkey Head Notes
During gameplay, press L3, L3, L3, L3, R3, R3, R3, R3, L3.
Scatter Gem Positions
During gameplay, press X, X, Left, Left, R3, R3, Right, Right.
The database at Game Oak contains more information regarding Amplitude for Sony PlayStation 2.