Context-dependent Encoding of Descending Neurons in Drosophila Open Access

Qiu, Jirui (2017)

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Animals exhibit a wide variety of behaviors. How these behaviors are initiated, sustained, and modulated, though, remains largely unknown. For a substantial number of animals, including Drosophila, descending neurons communicate signals from the brain to local circuits to generate motor outputs. However, little is known about the associations between descending neurons and behavior outputs. Now the combination of optogenetic techniques and an unsupervised measurement of animal behaviors permits us to study the behavioral effects of stimulating individual descending neurons in freely moving Drosophilae. Our study finds evidence supporting the theory that these induced behavioral effects are context-dependency. Previous behavioral statuses can affect the behavioral effects induced by activating individual descending neurons.
Our study applies mutual information as a main approach to measuring the correlations between behavioral probability distributions, and to characterizing context-dependency. To further explore the properties of context-dependency, we then apply multiple statistical methods to analyze the experimental results. Based on information theory, we study the general principles of the signal transmission within the neural network and possible theoretical hypotheses for the context-dependency. This thesis aims to provide insights into how the brain modulates animal behaviors, and pave the way toward a deeper understanding toward the functioning of the neural network.

Table of Contents

Chapter 1 Introduction
1.1 Introduction
1.2 Background
1.2.1 Drosophila
1.2.2 Descending neurons
1.2.3 Optogenetic Techniques
1.2.4 GAL4-UAS system
1.2.5 Behavioral analysis
1.3 Methods
1.3.1 Unsupervised Behavioral Analysis
1.3.2 Data Set
1.3.3 Mutual Information
1.3.4 Estimation of Mutual information
Chapter 2 Results
2.1 Context-dependent Behavioral Transitions
2.2 Shuffling
2.3 Estimating Mutual Information
2.4 Comparison of Behavioral Effects
Chapter 3 Discussion
3.1 Introduction
3.2 Theoretical Basis from Information Theory
3.2.1 Information Transmission within Neural Networks
3.2.2 Comparisons of Neural Networks with Varied Number of Layers
3.2.3 Trade-off between Sensitivity and Complexity
3.2.4 Context-dependency - An Approach to Integrating and Compensating the Loss of Information
3.3 Biological Basis of Context-Dependency
Chapter 4 Prediction and Experimental Design
4.1 Introduction
4.2 Inhibiting Neighboring Descending Neurons
4.3 Isolating Segments in Ventral Nerve Cord
4.4 Descending Neurons at Multiple Firing Rates
Chapter 5 Further Studies
5.1 Alternative quantifications of context-dependency
5.2 Context-dependency in Multiple Scales
5.3 Properties of the Communication Channels
5.4 Conclusion
Chapter 6 Summary
6.1 Results
6.2 Discussion
6.3 Conclusion

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