1. A visual prosthesis, comprising: an array of electrodes suitable to be placed in proximity to visual neural tissue; a camera for receiving a visual scene; and a control device coupling said camera to said array of electrodes including a neural stimulator for applying a plurality of stimuli visual neural tissue; means for measuring brightness responses to said stimuli; means for deriving a formula based upon said brightness responses; and means for generating stimuli by applying said formula to visual input from said camera.