Project Summary/Abstract An animal’s brain consists of interconnected neurons that are responsible for processing sensory information over many timescales to guide behavior. The function of that brain is determined partially by the animal’s connectome – the topology, strength, and valence of every connection in its brain. While a draft of the whole connectome for an animal (the worm Caenorhabditis elegans) has been available for decades, recent work has found that this connectome varies dramatically through development and between individuals. It is not stereotyped as expected. We do not know how this large variability in connectivity manifests itself in brain activity through development or between individuals. Nonetheless, the single connectome an animal has must somehow be able to support every behavior that the animal may perform in a given instant. Each of these behaviors may engage overlapping portions of the brain. This project aims to leverage calcium imaging to study how whole-brain activity in C. elegans varies through development and between individuals. Our goal is to clarify precisely how large-scale structure and function are related in a simple system. To do this, we will perform whole-brain imaging with cellular resolution in a collection of behaving individual animals as they progress from newly-hatched larvae through a series of molts and turn into adults. These long-term calcium recordings will be complemented by microfluidic measurements of whole-brain responses to chemosensory cues at multiple developmental stages. Throughout this process, we will focus on the following big questions: (1) How does the activity of every neuron in a worm change through development? (2) How does brain activity vary between genetically identical individuals? (3) How are each of the above questions affected by changes in environmental context? (4) How does the relationship between brain activity and connectivity change over development, and how does it vary between individuals? If successful, this work could provide unprecedented insight into how brain function changes as an animal adds neurons, connections, and synapses. It will show how inter-individual and intra-individual variation are related to the brain’s connectome. This will have immediate value in guiding expectations about how brain activity and brain wiring are related in other model systems and humans, where direct information about wiring is less readily available.