Elementary movement detectors in an insect visual system

In the theoretical part of the present work the input-output relation for a multi-input system is developed into a functional power series. This is formally equivalent to a decomposition of the system into a sum of all possible combinations of 1-, 2-, 3-... input subsystems. The average response of the system to a uniformly moving patern is known to be a Fourier series with respect to spatial frequency. The coefficients of the series are linear combinations of the weights by which different subsystems contribute to the total reaction. If a system can be shown to have essential nonlinearities of no higher than second order it is possible to calculate, from a Fourier analysis of the average movement response, the weight by which the nonlinear interaction between any two input elements contributes to the total reaction. This interaction is termed elementary movement detector. By the analysis presented here the arrangement of the elementary movement detectors may be determined for a two-dimensional array of input elements and the strength of their contributions to the total movement reaction may be calculated. Special experimental methods have been developed which allow one to apply this analysis to the visual system of the fruitfly Drosophila. The preliminary data presented show that the direction sensitive optomotor response of Drosophila can be attributed predominantly to the contributions from two elementary movement detectors which interconnect neighbouring visual elements. The detectors are oriented in the hexagonal array of the compound eye at +30° and at-30° with respect to the horizontal line of symmetry. A weak contribution from a detector between neighbouring elements along the horizontal line of symmetry is suggested by the present data. In the course of the analysis the contrast transfer properties of the compound eye are characterized.