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This book is written for our friends in Germany and abroad to give an account of our main interests during the last years. We gratefully take the opportunity provided by Springer Verlag with the Series Studies of Brain Function to present some of our recent work on vision in Drosophila in a coherent form and in a broader context than publications in periodicals would permit. We want to picture the visual system of the fly fitting it into the natural history from which our own vision emerged. In this perspective genetics seems to be a useful ap proach. Our study is a rather personal exercise. We have neither the expertise nor the ambition to cover the whole literature or to compileú all the knowledge about vision in flies. We are not even in a position to present, with the appropriate weight and breadth, the pioneering contributions of those who established this field of research. Our account mirrors our own intersects. We are selecting and interpreting other peoples´ work from this subjective point of view, although we try to present it in its own right. For several years we have been fascinated by the first glimpses of the "inner life" of an insect and we hope to convey this fascination to those who are not discouraged by the scien tific procedure.

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Initiating Activity.- Input-Output Analysis.- Orientation and Orientedness.- Neurogenetics.- A: Eye, Brain, and Simple Behavior.- 1 The Compound Eye.- The Distribution of Optical Axes.- Neural Superposition.- The Equator.- Angular Sensitivity of Visual Elements.- Screening Pigments.- Electrical Responses to Light.- Spectral Sensitivity.- Polarization Sensitivity.- Ocelli.- Summary.- 2 Neuronal Architecture of the Visual System.- Retinotopic Maps.- Visual Foci.- Lamina ganglionaris.- Medulla, Lobula, Lobula Plate.- 3 Motion Sensitivity Under Open Loop Conditions.- Orientation.- Optomotor Yaw Torque and Turning Behavior.- Optomotor Equilibrium.- Optomotor Roll and Pitch Responses.- Translatory Optomotor Responses.- Optomotor Posture Control by Visual Motion.- Visually Induced Start of Flight and Landing.- 4 Toward Correlating Structure and Function.- Retinal Subsystems.- Color Vision.- Polarization Sensitivity in Behavior.- Dark Adaptation on Account of Acuity.- Elementary Movement Detectors.- 3H-Deoxy-Glucose.- Lobula Plate Giant Neurons.- Other Visual Pathways.- B: The Behavioral Structure of the Visual System.- 5 Flying Straight.- Artificial Closed Loop Conditions.- Optomotor Balance.- Closed Loop Gain of Freely Rotating Flies.- 6 Endogenous Behavior in Yaw Torque Fluctuations.- Saccades and Torque Spikes.- Torque Spikes as Fixed Action Patterns.- Reafference Control.- Spontaneous Occurrence of Torque Spikes.- Torque Spikes Rules.- Flight Modes.- 7 Orientation Toward Objects.- Fixation Behavior in Musca, a Theory.- The Object Response in Drosophila.- Special Role of Front-to-Back Motion.- Range of Stimulus Parameters for the Position Function.- Two Responses Contribute to Yaw Torque.- Analysis of Individual Torque Traces.- Mutants with Altered Orientation Behavior.- Fixation in Flight.- Fixation in Walking Flies.- 8 Menotaxis.- Small Angle Oscillations.- Significance of Reafferent Stimuli.- The Object Response During Menotaxis.- 9 Foreground-Background Experiments.- Figure-Ground Discrimination in Musca.- One Output-Multiple Input Systems.- A Simple Model Generating Figure-Ground Discrimination.- Figure-Ground Discrimination in Drosophila.- Tracking.- Interplay Between Object Response and Optomotor Balance.- Dissociation into Object and Background.- Interpretation of Object Response.- Can the Musca Theory on Pattern-Induced Flight Orientation Be Applied to Drosophila?.- 10 Visual Control in Free Flight.- 11 Selective Attention.- Displacement Experiments.- Open Loop Oscillations.- 12 Plasticity of Visuo-Motor Coordination.- 13 Valuation.- Hypotheses.- Significance of Simple Visual Stimuli.- Context Dependence.- Visual Components of Courtship Behavior.- Conditioning.- Synopsis.- The Three Levels of Orientation.- Parallel Processing.- Actions and Responses.- Voluntary Behavior.- Orientedness Revisited.- Appendix 1: List of Neurological Mutants.- Appendix 2: Symbols, Dimensions, Abbreviations.- References.

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