Arani Roy, Jason J Osik, Neil J Ritter, Shen Wang, James T Shaw, József Fiser, Stephen D Van Hooser

Many circuits in the mammalian brain are organized in a topographic or columnar manner. These circuits could be activated - in ways that reveal circuit function or restore function after disease - by an artificial stimulation system that is capable of independently driving local groups of neurons. Here we present a simple custom microscope called ProjectorScope 1 that incorporates off-the-shelf parts and an LCD projector to stimulate surface brain regions that express channelrhodopsin-2 (ChR2). In principle, local optogenetic stimulation of the brain surface with optical projection systems might not produce local activation of a highly interconnected network like the cortex, due to potential stimulation of axons of passage or extended dendritic trees. However, here we demonstrate that the combination of virally-mediated ChR2 expression levels and light intensity of ProjectorScope 1 is capable of producing local spatial activation with a resolution of about 200-300µm. We use the system to examine the role of cortical activity in the experience-dependent emergence of motion selectivity in immature ferret visual cortex. We find that optogenetic cortical activation alone - without visual stimulation - is sufficient to produce increases in motion selectivity, suggesting the presence of a sharpening mechanism that does not require precise spatiotemporal activation of the visual system. These results demonstrate that optogenetic stimulation can sculpt the developing brain.