Activity of descending contralateral movement detector neurons and collision avoidance behaviour in response to head-on visual stimuli in locusts

We recorded the activity of the right and left descending contralateral mov ement detectors responding to 10-cm (small) or 20-cm (large) computer-gener ated spheres approaching along different trajectories in the locust's front al field of view. In separate experiments we examined the steering response s of tethered flying locusts to identical stimuli. The descending contralat eral movement detectors were more sensitive to variations in target traject ory in the horizontal plane than in the vertical plane. Descending contrala teral movement detector activity was related to target trajectory and to ta rget size and was most sensitive to small objects converging on a direct co llision course from above and to one side, Small objects failed to induce c ollision avoidance manoeuvres whereas large objects produced reliable colli sion avoidance responses. Large targets approaching along a converging traj ectory produced steering responses that were either away from or toward the side of approach of the object, whereas targets approaching along trajecto ries that were offset from the locust's mid-longitudinal body axis primaril y evoked responses away from the target. We detected no differences in the discharge properties of the descending contralateral movement detector pair that could account for the different collision avoidance behaviours evoked by varying the target size and trajectories. We suggest that descending co ntralateral movement detector properties are better suited to predator evas ion than collision avoidance.