Adaptive collision avoidance behaviours require accurate detection of complex spatiotemporal properties of an object approaching in an animal's natural, 3-dimensional environment. Within the locust, the lobula giant movement detector (LGMD) and its postsynaptic partner, the descending contralateral movement detector (DCMD) respond robustly to images that emulate an approaching 2-dimensional object and exhibit firing rate modulation correlated with changes in object trajectory. It is not known how this pathway responds to visual expansion of a 3-dimensional object or an approaching object that changes velocity, both of which representing natural stimuli. We compared DCMD responses to images that emulate the approach of a sphere with those elicited by a 2-dimensional disc. A sphere evoked later peak firing and deceased sensitivity to the ratio of the half size of the object to the approach velocity, resulting in an increased threshold subtense angle required to generate peak firing. We also presented locusts with a sphere that decreased or increased velocity against either a white or flow field background. A velocity decrease resulted in transition-associated peak firing followed by a firing rate increase that resembled the response to a constant, slower velocity. A velocity increase resulted in an earlier increase in the firing rate that was more pronounced with an earlier transition. For the flow field contrast used here, we observed no effect of background motion on responses to approaches along constant or changing velocities. These results further demonstrate that this pathway can provide motor circuits for behaviour with salient information about complex stimulus dynamics.

Stott et al Python codePython code to calculate stimulus parameters of a disc and sphere based on experimenter input to GUI.Stott et al Matlab codeMatlab code to autodetect DCMD firing parameters