RESPONSES TO OBJECT APPROACH BY A WIDE-FIELD VISUAL NEURON, THE LGMD2OF THE LOCUST - CHARACTERIZATION AND IMAGE CUES

The LGMD2 belongs to a group of giant movement-detecting neurones whic h have fan-shaped arbors in the lobula of the locust optic lobe and re spond to movements of objects. One of these neurones, the LGMD1, has b een shown to respond directionally to movements of objects in depth, g enerating vigorous, maintained spike discharges during object approach . Here we compare the responses of the LGMD2 neurone with those of the LGMD1 to simulated movements of objects in depth and examine differen t image cues which could allow the LGMD2 to distinguish approaching fr om receding objects. In the absence of stimulation, the LGMD2 has a re sting discharge of 10-40 spikes s(-1) compared with <1 spike s(-1) for the LCMD1. The most powerful excitatory stimulus for the LGMD2 is a d ark object approaching the eye. Responses to approaching objects are s uppressed by wide field movements of the background. Unlike the LGMD1, the LGMD2 is not excited by the approach of light objects; it specifi cally responds to movement of edges in the light to dark direction. Bo th neurones rely on the same monocular image cues to distinguish appro aching from receding objects: an increase in the velocity with which e dges of images travel over the eye; and an increase in the extent of e dges in the image during approach.