On the building of binary spelling interfaces for augmentative communication

A criterion for design of optimal binary spelling interfaces (SI)-the average expectation of number of writing steps (trials) required to write one letter-is presented and discussed. This criterion is relevant for practical usage of any menu-oriented alternative communication system, mechanical device or brain-computer-interface, when a user (typically, a patient with devastating neuromuscular handicap) can not create an intended single binary response with an absolute reliability. An algorithm for building of a corresponding binary tree is developed and evaluated. This algorithm is efficient, when selection probabilities have essentially different values (the worst case).