Current spread and channel interaction

Because monopolar stimulation in the conductive perilymph and bone produces a broad electric field, current from one electrode excites neurons far along the tonotopic axis, so nominally separate channels overlap. This channel interaction — measurable as broad spread-of-excitation functions and forward-masking curves — is the principal reason a 22-electrode implant delivers only roughly 4-8 effectively independent channels of information. It, more than the coding algorithm, sets the CI performance ceiling.

Interaction has a spatial component (overlapping current fields) and a temporal component (residual polarization, refractoriness, and neural adaptation when nearby sites fire in quick succession); interleaving pulses addresses the temporal but not the spatial part. Mitigations include current focusing (tripolar, phased-array, current-focusing), current steering (weighting adjacent electrodes to create virtual channels between contacts), and, more radically, moving to physically confined stimulation such as optical or intraneural approaches.