Encoder–decoder brain pair

The minimal BBI unit is two participants with fixed, asymmetric roles: an encoder (sender) whose brain activity is read to generate a message, and a decoder (receiver) into whose brain the message is written. In animal work the roles are literal — Pais-Vieira et al. (2013) trained an encoder rat on a discrimination task and delivered its somatosensory or motor activity as microstimulation to a decoder rat that had to reproduce the choice. In human work the sender uses motor imagery or SSVEP and the receiver's cortex is stimulated by TMS.

The asymmetry is a defining property, not an accident of early hardware. Reading and writing to the brain are very different operations with very different maturity, so performance is capped by the weaker side — the decoder animal typically performs well above chance but far below the encoder. Bidirectional or peer-to-peer linking, in which both brains read and write at once, is largely untried and would multiply the unsolved write-side problems.