Processing of optical information by the visual system of the fly.

A detailed understanding of the principles of sensory information processing by the nervous system require investigations with a variety of "model" systems at different levels of complexity. The fly as a model system represents a compromise in the sense that the properties of information processing are taking place in a rather complex nervous system that is amenable to a quantitative analysis at three different levels: at the phenomenological level the overall function of the system, its input-output behaviour, and its logical organization are studied. At the second level, the functional principles of the subsystems, expressed by its algorithmic properties are the object of the analysis. At the third level the investigation focusses on the individual nerve cells and the neuronal circuitry involved in the computations. The approach, reviewed here, deals with an analysis of the visio-motor control system of the fly at the phenomenological level which leads to an algorithmic description of the main subsystems for extraction of motion- and position-information. The analysis at the third level is connected with the investigation of the so called figure-ground and pattern discrimination problem by the visual system of the fly. In this connection the elements of a new movement detector theory are mentioned.