REVIEW Visually guided orientation in flies: case studies in computational neuroethology Received: 7 January 2003 / Revised: 10 April 2003 / Accepted: 11 April 2003 / Published online: 15 May 2003 Ó Springer-Verlag 2003 Abstract To understand the functioning of nervous systems and, in particular, how they control behaviour we must bridge many levels of complexity from mole- cules, cells and synapses to perception behaviour. Al- though experimental analysis is a precondition for understanding by nervous systems, it is in no way suf- ficient. The understanding is aided at all levels of com- plexity by modelling. Modelling proved to be an inevitable tool to test the experimentally established hypotheses. In this review it will by exemplified by three case studies that the appropriate level of modelling needs to be adjusted to the particular computational problems that are to be solved. (1) Specific features of the highly virtuosic pursuit behaviour of male flies can be under- stood on the basis of a phenomenological model that relates the visual input to the motor output. (2) The processing of retinal image motion as is experienced by freely moving animals can be understood on the basis of a model consisting of algorithmic components and components which represent a simple equivalent circuit of nerve cells. (3) Behaviourally relevant features of the reliability of encoding of visual motion information can be understood by modelling the transformation of postsynaptic potentials into sequences of spike trains. Introduction Brains are believed to belong to the most complex structures in the universe. They consist of densely packed and intricately interconnected networks of neurons, each of which has already highly complex functional properties. With their neuronal machinery even relatively small animals are able to do extraordi- nary things—at least if judged by comparison with man- made artificial systems. Think, for instance, of a hoverfly hovering in front of an object, suddenly sweeping to the side at a high velocity but returning within seconds to the same spot, or of the acrobatic flight manoeuvres of a male blowfly while pursuing another fly in the context of mating behaviour. During such astonishing manoeuvres information about the environment has to be gathered by the sense organs, processed rapidly by the nervous system, adjusted according to internally stored infor- mation, transformed into motor commands and even- tually used to guide behaviour. To understand the functioning of nervous systems and, in particular, how they control behaviour we must bridge many levels of analysis from molecules, cells and synapses to perception and behaviour. Although experimental analysis is a precondition for under- standing information processing by nervous systems, it is in no way sufficient. In the 18th century the Italian philosopher Giambattista Vico proposed the principle that we can only understand what we make. Translat- ing this principle to the study of brain function it means that in order to understand the brain we must ‘construct’ one and simulate the behaviour of the organism. Modelling brain function always entails the problem of the level of organisation at which the rel- evant features of the system can be grasped most appropriately. For instance, trying to model the behavioural performance of an entire animal on the basis of all molecules making up the involved nerve cells would be not only impossible but also an absurd encounter. Instead, a more promising approach is to model, and in this way to try to understand, the functioning of nervous systems via a series of pro- gressively reductive levels of explanation. These levels range from a phenomenological characterisation of the performance of the entire system to a description of the biophysical properties of nerve cells and their synaptic J Comp Physiol A (2003) 189: 401–409 DOI 10.1007/s00359-003-0421-3 M. Egelhaaf Æ N. Bo¨ddeker Æ R. Kern Æ J. Kretzberg J. P. Lindemann Æ A.-K. Warzecha M. Egelhaaf (&) Æ N. Bo¨ddeker Æ R. Kern Æ J. Kretzberg J. P. Lindemann Æ A.-K. Warzecha Lehrstuhl fu¨r Neurobiologie, Fakulta¨t fu¨r Biologie, Universita¨t Bielefeld, Postfach 10 01 31, 33501 Bielefeld, Germany E-mail: martin.egelhaaf@uni-bielefeld.de Tel.: +49-521-1065570 Fax: +49-521-1066038