Primate brains contain cortical areas that exhibit selective engagement in high-level sensory or behavioral operations. The functional specialization of these regions is thought to be central to primate-specific cognitive faculties and to associated disorders. Deciphering the origins of functional specialization in primate brain regions has been an enormously challenging task, however, due in large part to the absence of suitable experimental tools. To address this problem, we will develop a method for measuring the activity of inputs to specialized areas from throughout the brain, permitting systematic analyses of information flow in the multiregional neural circuitry that gives rise to high-level functions. Our method will employ a conceptually new family of genetically encoded imaging probes called NOSTICs, which transduce the calcium signaling of NOSTIC-expressing neurons into localized hemodynamic signals that can be dynamically monitored using brain-wide measurement techniques like functional magnetic resonance imaging (fMRI). When delivered using retrogradely transported viral vectors, NOSTICs can permit targeted fMRI-based recording of neural activity in distributed cell populations that provide monosynaptic input to any injection target in the brain. In our preliminary work, we have created first-generation NOSTIC probes and used them to demonstrate genetically targeted functional imaging in rodents. In Aim 1 of this project, we will take two steps that adapt this tool for use in nonhuman primates. We will create second- generation NOSTICs that display improved performance for circuit-specific functional imaging, while also devel- oping viral vectors that allow expression of these probes to be tracked longitudinally in primate brains. We will also adapt the NOSTIC probes for incorporating into adeno-associated viruses, which provide extended capa- bility compared with the herpes viruses we currently use. In Aim 2, we will perform pilot experiments to investigate whether NOSTICs can provide circuit-specific readouts in nonhuman primates. These tests will already be pos- sible using our currently available probes and vectors, and new variants from Aim 1 will also be tested when available. Successful demonstration of NOSTIC functionality for circuit imaging in marmosets constitutes our proposed go/no-go criterion for entry into the UH3 stage of this project. Then in Aim 3 (UH3 stage), we will validate NOSTIC probes in two paradigms that explore their performance across brain regions, experimental contexts, and primate species. In the first paradigm, we will apply NOSTICs to examine origins of functional specialization in face-selective regions of the marmoset brain. In the second paradigm, we will apply NOSTICs to investigate brain-wide contributions to object selective responses in the ventral stream of the macaque visual cortex. These experiments will be performed as multi-laboratory collaborations that both harness and dissemi- nate the NOSTIC technology; this work will therefore establish a broadly applicable transformative approach for mechanistic analysis of primate brain function.

The cognitive capabilities of primates depend on integrated interactions among cells and brain areas that cur- rently cannot be studied at a whole-brain level in live animals. In this project, we will adapt and validate a un- precedented experimental approach called hemogenetic imaging, which permits brain-wide functional measure- ments from genetically targeted cell types and circuit elements. Through technology development and multi- laboratory collaboration, this project will establish transformative and broadly applicable methodology for inves- tigation of primate brain function.