Neuronal processing of behaviourally generated optic flow: experiments andmodel simulations

The stimuli traditionally used for analysing visual information processing are much simpler than what an animal sees when moving in its natural enviro nment. Therefore, we analysed in a previous study the performance of an ide ntified neuron in the optomotor system of the fly by using as visual stimul i image sequences that were experienced by the animal while walking in a st ructured environment. These electrophysiological experiments revealed that the fly visual system computes from behaviourally generated optic flow a ra ther unambiguous representation of the animal's self-motion. In contrast to conclusions based on simple stimuli, the directions of turns are represent ed by an interneuron, the HSE cell, quite independent of the spatial layout of the environment and its textural properties when the cell is stimulated with behaviourally generated optic flow. This conclusion is substantiated here by further experimental evidence. Moreover, it is shown that the large ly unambiguous responses of the HSE cell to behaviourally generated optic f low can be replicated to a large extent by a network model of the fly's vis ual motion pathway. These results stress the significance of naturalistic s timuli for analysing what is encoded by neuronal circuits under natural ope rating conditions.