Humans are highly sensitive to the overall shape of objects—a critical step in object identification. One hypothesis is that the brain uses the shape of the spread of stimulus-evoked activity across the surface of visual cortex in order to process shape, exploiting retinotopy—the well-documented organization of neurons as a projection of the visual field across the surface of the cortex. Prior work from our group has found that the shape of activity across the cortex holds information that can be used to decode the overall shape of a stimulus, but whether the brain actually uses this information is as of yet unknown. The current proposal aims to test the hypothesis that the shape of activity across the cortex can encode shape in vision by seeing if the perception of shape can be brought about by artificially evoking shaped activity in visual cortex. I will evoke this shaped neuronal activity using optogenetics, a technique for artificially exciting neurons using light. By controlling the pattern of excitation light, I can control the shape of the activity I evoke. If I can use this optogenetic stimulation to bias macaque monkeys performing a shape discrimination task to report seeing the shapes corresponding to the artificially inserted activity, it would indicate that the brain relies on the retinotopic shape of activity across the cortex to process shape in vision. Experiments in Aim 1 will describe the behavior of macaques performing a simplified shape discrimination task and the neuronal signals recorded as they perform the task, with detailed computational modeling of potential neuronal strategies. Aim 2 will be focused on the development of the pipeline necessary for precise shaping of optogenetically evoked activity. Aim 3 will be an experiment in which optogenetics will be used to introduce artificial activity of a specific shape into the visual cortex of macaques performing the shape discrimination task. I will see if behavior, and thus likely perception, is systematically affected by the addition of artificial neuronal activity that only contains information by virtue of the shape of its spread across the cortex. The proposed work will be an important contribution to our understanding of how shape is processed in vision and will have broad implications for the utility of patterned optogenetic stimulation in translational contexts such as for use in cortical neuroprostheses for restoring vision.