Colour inputs to motion and object vision in an insect

Abstract

Self-motion detectors of the dragonfly ventral cord have large fields that are sensitive to whole-field one- way motion with little habituation to the motion. In contrast, object-motion detectors have large fields, but their responses to the motion of small objects within the field habituate more easily and some are non-directional. Both types are large neurons that can be used for electrophysiological recording for long periods and are previously described in their anatomy and their responses to black and white moving patterns (Oldberg 1986). When tested with a flash of controlled intensity and wavelength, the self-motion neurons have a spectral sensitivity similar to that of photoreceptors with a peak in the green near 500 nm, but when tested with a moving edge, they have a single peak near 560 nm. They behave towards a pattern of two colours as if they are colour blind. Similar results are known for the bee and some butterflies. Forward-looking object-motion neurons have a spectral sensitivity curve that is rather flat from 380 nm to 580 nm, some with a peak in the UV. When they are tested with a moving coloured pattern on a differently coloured background, there is no value of the foreground brightness, which gives a null response, however the brightness is adjusted. The optimum response to a coloured object appears when the background is green and the object is another colour; usually blue is the most effective. Discrimination of a small object is less effective when the object is green. These results suggest that colour vision is associated with object-vision; and that object-motion detectors are not colour blind, but do not necessarily discriminate colours.