Retinitis pigmentosa and age-related macular degeneration are two of the more frequent causes of blindness in the developed world.1-3 Both diseases are progressive and begin with the degeneration of photoreceptors. In later stages of these diseases, bipolar, amacrine, and ganglion cells are still present, though their numbers are significantly decreased4-6 and their spatial organization and circuitry are significantly disorganized.7,8 There are more than 180 different gene mutations that result in photoreceptor diseases for which there is currently no cure or treatment.1 Ideally, it would be possible to develop a treatment for these conditions that would not require targeting each genetic defect independently. Several groups are developing implantable microelectronic visual prostheses that produce percepts by electrically stimulating remaining retinal neurons. To date, several groups have succeeded in generating visual percepts via electrical stimulation with implanted acute, semi-acute, and long-term retinal prostheses in human patients.9-14 The ultimate goal of these projects is to generate useful vision in blind patients by transforming a video stream into a spatial and temporal sequence of electrical pulses that represents meaningful visual information. However, creating a perceptually meaningful pattern of stimulation is dependent on a detailed understanding of the perceived intensity of any given stimulation pattern; to date, the literature examining the perceptual consequences of electrical stimulation remains relatively sparse.10-12,15-20 A visual prosthesis should produce regions of constant brightness across a range of brightness levels, and ideally these brightness levels should be consistent with the apparent brightness of objects as they appear to those with normal vision. Our goal was to examine how apparent brightness changes as a function of stimulation intensity in two blind human subjects chronically implanted with a prototype epiretinal prosthesis consisting of a 4 × 4 array of 16 stimulating electrodes. In experiment 1, subjects rated the apparent brightness of pulse stimuli on individual electrodes using a reference pulse of fixed amplitude. We found that apparent brightness as a function of current amplitude can be described using a simple power function. In experiment 2, a brightness-matching technique was used to compare apparent brightness across pairs of electrodes. We found that the apparent brightness of a given electrode can be related to other electrodes on the array using the same simple power function model. The results from these two experiments suggest that a relatively simple model for scaling current across electrodes may, to a first approximation, be capable of producing equivalently bright phosphenes across an entire array.