Projected fields
The perceived location and spatial extent on the body of a percept evoked by microstimulation of somatosensory cortex. Because cortex is somatotopically organized, stimulating a given electrode projects its percept to the skin region that electrode represents, and a fixed electrode tends to project to a consistent location across days and months. Reported sizes range from a fingertip to most of a finger.
Stable projected fields are what make an implant usable as an intuitive interface: finger identity can be encoded simply by which electrode is active, a place code that the user reads without effort. Characterizing the projected field of every electrode, its location, size and reliability, is therefore an early step in commissioning a somatosensory ICMS array.
Mapping an array yields a table: electrode 7 projects to the thumb pad, electrode 12 to the index fingertip, electrode 19 to the palm; the decoder later routes touch from a prosthetic thumb sensor to electrode 7.
The projected-field map is the routing table between prosthetic sensors and cortical electrodes.