First level:
Pause at the zone selection screen. Press L1, R1, L1, R1.
Second level:
Pause at the zone selection screen. Press L2, R2, L2, R2.
Third level +
Pause at the zone selection screen. Hold L1 + R1 + L2 + R2.
Original Pong:
At the options screen, press L1, R1, L1, R1, L1, R1 then select the star field.
