The photo-sonic cells were a component of the Dalek casing by the time the Grey Daleks were in wide use by the Dalek Empire. It was a flat slab of hardware contained between the dome section and srating section, thus resting directly below the combat computor and above the the environment chamber housing the Dalek creature.

The photo-sonic cells of the casing marked as "D" on a Space Intelligence Service diagram of Dalek anatomy. (NOTVALID: Inside a Dalek)

The photo-sonic cells recorded all visual and auditory data taken in by the Dalek; it could be retrieved at will and almost instnalty. (NOTVALID: Inside a Dalek)

= Behind the scenes

The photo-sonic cells play the same part in Dalek anatomy as the databanks later seen on television in Into the Dalek.