PROJECT SUMMARY ABSTRACT Understanding how brains process information and how dysfunction disrupts that processing requires measuring patterns of neural activity with cellular spatial and millisecond temporal resolution. These patterns are by nature distributed: many brain regions contribute to any given act of perception, cognition, or action. Adding to this complexity is both neuronal diversity and plasticity: each region contains neurons of many different cell types, each with distinct physiology and anatomy, and neural activity patterns evolve throughout the lifespan. The complexity of information processing in the brain demands technologies that can record at millisecond timescales throughout a region, spanning both different parts of the region as well as all layers, to understand how local or columnar information processing relates to more distributed information processing. We further need to be able to record at high density in not just one region, but across a set of regions in a circuit, and we need to be able to combine these high-density recordings with powerful imaging approaches that can provide information about cell types and related variables. We have developed flexible polymer probe technology that has the potential to address all of these needs, but additional development is required to fully realize the potential of these devices and to bring them and the associated hardware and software into broad and routine neuroscience practice. Our Aims are 1) To refine and distribute existing high-density polymer electrode arrays, and 2) To develop and distribute smaller, lighter, higher density polymer electrode arrays and electronics that allow for exceptionally high-density recordings across brain regions and species. We have assembled a core team of scientists and engineers with expertise spanning polymer electrode technology, integrated circuit design, real-time systems, large-scale recording, imaging and commercial experience. We have also brought together a set of outstanding scientists to develop surgical approaches for new model systems and applications, to test devices across many brain areas and animal models, and to combine polymer devices with imaging. The resulting technology will enable new generations of experiments that will bring us closer to understanding information processing in the brain.