We are grateful to Johannes Felsenberg who has been supporting this project throughout with his expertise and fruitful ideas. We thank Anna-Maria Jürgensen for her contribution to the initial model implementation. We thank Bertram Gerber and Anna-Maria Jürgensen for insightful comments on an earlier version of this manuscript.
Extinction learning, the ability to update previously learned information by integrating novel contradictory information, is a key mechanism for adapting our behavior and of high clinical relevance for therapeutic approaches to the modulation of maladaptive memories. Insect models have been instrumental in uncovering fundamental processes of memory formation and memory update. Recent experimental results in Drosophila melanogaster suggest that, after the behavioral extinction of a memory, two parallel but opposing memory traces coexist, residing at different sites within the mushroom body. Here we propose a minimalistic circuit model of the Drosophila mushroom body that supports classical appetitive and aversive conditioning and memory extinction. The model is tailored to the existing anatomical data and involves two circuit motives of central functional importance. It employs plastic synaptic connections between Kenyon cells and mushroom body output neurons (MBONs) in separate and mutually inhibiting appetitive and aversive learning pathways. Recurrent modulation of plasticity through projections from MBONs to reinforcement-mediating dopaminergic neurons implements a simple reward prediction mechanism. A distinct set of four MBONs encodes odor valence and predicts behavioral model output. Subjecting our model to learning and extinction protocols reproduced experimental results from recent behavioral and imaging studies. Simulating the experimental blocking of synaptic output of individual neurons or neuron groups in the model circuit confirmed experimental results and allowed formulation of testable predictions. In the temporal domain, our model achieves rapid learning with a step-like increase in the encoded odor value after a single pairing of the conditioned stimulus with a reward or punishment, facilitating single-trial learning.
Located in Northern Virginia along the western bank of the Potomac River, Fairfax County is the largest jurisdiction in the Washington DC, metropolitan area with a population of over one million. With the service area of 234 square miles and over 850,000 customers, it has one of the nation's largest sanitary sewer system which include 3300 miles of sanitary sewer ranging from 8 inches to 60 inches, 64 pumping stations, and 53 permanent flow meters. The oldest sewers were installed in the 1940s and 1950s. Older lines were constructed using vitrified clay pipe. Lines installed between 1950 and 1970 were primarily concrete and lines installed after 1970 were made from PVC. Approximately 100 millions gallons of wastewater is conveyed daily to six treatment facilities. Although sanitary sewer overflows (SSOs) in Fairfax County were not excessive, these were also not within a generally acceptable range. Wastewater Collection Division (WCD) began tracking SSOs closely in 1995, when there were 128 occurrences. There were no known quality violations as a result of SSOs, and 22 sewer damage claims were paid to private property owners. WCD's primary reason for embarking on major operational and management changes was based on its desire to improve customer service and to prepare for the anticipated EPA SSO control regulations, which at the time were in the early stages of development. In an effort to reduce SSOs, a comprehensive approach was adapted which included, among others, the following initiatives: reorganization, employee participation, streamlining the workload, thorough investigation of all SSOs, sewer system rehabilitation/ replacement, purchase of new equipment and implementation of new business practices. This paper will discuss all these initiatives in detail to illustrate how their implementation, optimization and integration has resulted in very significant reduction in sanitary sewer backups in private properties and overflows in surface waters. The case study will also present the program results using statistical data collected over the course of several years.
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