EPOC X can be operated with the rubber comfort pads in the M1/M2 sensor positions, as well as the P3/P4 positions. To switch to mastoid referencing from the default, take the rubber pads out from the M1/M2 sensors and insert them into P3/P4 sensor positions. Ensure wet felt pads on all other sensors including M1/M2 to ensure mastoid referencing. Please see image below as reference.