PROJECT SUMMARY/ ABSTRACT Electrical stimulation of visual areas in the brain generates the perception of dots of light (‘phosphenes’), even after decades of blindness. Clinical brain-computer interfaces offer a possible approach for restoring vision in blind individuals with extensive damage to the eye and/or optic nerve, bypassing the retina. Ideally, stimulation on a given number of electrodes would elicit an equivalent number of discrete phosphenes, where the location and duration of each phosphene are predictable and controllable, forming an image akin to the letters on a matrix-board along the highway. However, previous studies show that simultaneous stimulation on multiple electrodes often evokes a single, merged phosphene at an unpredictable location (‘phosphene fusion’). Thus, we first have to address the critical need to develop a rigorously characterized, data-driven model that accurately predicts the perceptual experience evoked by a wide variety of spatiotemporal patterns of stimulation. We propose developing detailed, empirically driven, behaviorally validated models of neuronal activation during stimulation on multiple electrodes to understand phosphene fusion. We will record electrophysiology data from a large area (12 cm2) of the monkey visual cortex bilaterally via 1024 intracortical Utah-array electrodes simultaneously, during microstimulation on single and multiple electrodes, eliciting phosphenes across the central 10-15 dva of each visual hemifield (i.e. 20-30 dva bilaterally), spanning foveal to peripheral regions. We will use phosphene localization and 2AFC match-to-sample tasks to obtain behavioral reports of phosphene characteristics and the occurrence of phosphene fusion. We will use our neuronal recordings and behavioral reports to develop and validate a detailed, biologically realistic model of current interactions, V1 and V4 neuronal activity, and perception, during microstimulation on multiple electrodes. As a result, we will understand and predict how the distance between electrodes in the cortex, the time interval between stimulation trains, and the stimulation of different visual areas (V1 and V4) give rise to distinct versus fused phosphenes, laying a crucial foundation for the generation of reliably interpretable phosphene images.