Behavioral Responses to a Repetitive Visual Threat Stimulus Express a Persistent State of Defensive Arousal in Drosophila
The neural circuit mechanisms underlying emotion states remain poorly understood. Drosophila offers powerful genetic approaches for dissecting neural circuit function, but whether flies exhibit emotion-like behaviors has not been clear. We recently proposed that model organisms may express internal states displaying "emotion primitives," which are general characteristics common to different emotions, rather than specific anthropomorphic emotions such as "fear" or "anxiety." These emotion primitives include scalability, persistence, valence, and generalization to multiple contexts. Here, we have applied this approach to determine whether flies' defensive responses to moving overhead translational stimuli ("shadows") are purely reflexive or may express underlying emotion states. We describe a new behavioral assay in which flies confined in an enclosed arena are repeatedly exposed to an overhead translational stimulus. Repetitive stimuli promoted graded (scalable) and persistent increases in locomotor velocity and hopping, and occasional freezing. The stimulus also dispersed feeding flies from a food resource, suggesting both negative valence and context generalization. Strikingly, there was a significant delay before the flies returned to the food following stimulus-induced dispersal, suggestive of a slowly decaying internal defensive state. The length of this delay was increased when more stimuli were delivered for initial dispersal. These responses can be mathematically modeled by assuming an internal state that behaves as a leaky integrator of stimulus exposure. Our results suggest that flies' responses to repetitive visual threat stimuli express an internal state exhibiting canonical emotion primitives, possibly analogous to fear in mammals. The mechanistic basis of this state can now be investigated in a genetically tractable insect species.
© 2015 Elsevier. Under an Elsevier user license. Received: December 9, 2014; Revised: March 3, 2015; Accepted: March 30, 2015; Published: May 14, 2015. We thank Allan Wong, Brian Duistermars, Kiichi Watanabe, Hidehiko Inagaki, Barret Pfeiffer, Prabhat Kunwar, Moriel Zelikowsky, Weizhe Hong, and Kenta Asahina for helpful comments and discussion. We also thank Gerald M. Rubin and Kevin Moses for financial support. Although all data in the manuscript were collected at Caltech, this project was initiated under the Janelia Farm Visiting Scientist Program, which provided funds for the initial prototype of the instrumentation and for associated software development. We are grateful to HHMI, NIH, and the Gordon and Betty Moore Foundation for financial support. This work was supported in part by NIH grant RO1-DA031389. W.T.G. is a fellow of the Jane Coffin Childs Foundation for Medical Research. We thank J.L. Anderson for comments on the model. Author Contributions: W.T.G., M.M.M., R.R.D., C.F., P.D.F., P.P., and D.J.A. performed research. D.J.A. performed the original, food-based pilot studies of the shadow response assay in Martin Heisenberg's laboratory and at Janelia Farm and commissioned the construction of the original shadow paddle apparatus at Janelia Farm. M.M.M. built the tracker, which was modified by P.D.F. under the supervision of P.P. T.T. built the original prototype for the shadow response apparatus. L.R. designed the custom software used to control the apparatus. W.T.G. performed the majority of the experiments discussed in this paper and did the majority of the modeling and analysis used in the paper, based on an initial model of a leaky integrator made by P.P. W.T.G., M.M.M., P.P., and D.J.A. designed research. W.T.G. built the apparatus used for these experiments, with engineering support from Caltech and Janelia Farm/HHMI. W.T.G., C.F., and M.M.M. programmed scripts for computational analysis of the data. D.J.A. and P.P. supervised research. W.T.G. and D.J.A. wrote the paper. SUPPLEMENTAL INFORMATION: Supplemental Information includes Supplemental Experimental Procedures, seven figures, and seven movies and can be found with this article online at http://dx.doi.org/10.1016/j.cub.2015.03.058.
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