1. A visual stimulation method, comprising: generating a first series of stimulation patterns by stimulating neural tissue of a patient with a series of impulsive electrical signals with varying stimulation amplitude; determining a relationship of stimulation amplitude to induced phosphene brightness in response to said first series of stimulation patterns; generating a second series stimulation patterns by stimulating neural tissue of a patient with a series of impulsive electrical signals, wherein said second stimulation pattern alters frequency; predicting how induced phosphene brightness depends on the second series of stimulation patterns by estimating perceptual threshold as an exponential function of stimulation frequency versus stimulation amplitude based on stimulation with a time constant of less than 1 ms; and altering further stimulation amplitude and frequency based upon a prediction derived from the step of predicting.