  Current steering on a multi-electrode array is commonly used to shape the
electric field in the neural tissue in order to improve selectivity and
efficacy of stimulation. Previously, simulations of the electric field in
tissue required separate computation for each set of the stimulation
parameters. Not only is this approach to modeling time-consuming and very
difficult with a large number of electrodes, it is incompatible with real-time
optimization of the current steering for practical applications. We present a
framework for efficient computation of the electric field in the neural tissue
based on superposition of the fields from a pre-calculated basis. Such linear
algebraic framework enables optimization of the current steering for any
targeted electric field in real time. For applications to retinal prosthetics,
we demonstrate how the stimulation depth can be optimized for each patient
based on the retinal thickness and separation from the array, while maximizing
the lateral confinement of the electric field essential for spatial resolution.
