The lobula giant motion detector (LGMD) in the locust visual system is a wide-field, motion-sensitive neuron that responds vigorously to objects approaching the animal on a collision course. We investigated the computation performed by LGMD when it responds to approaching objects by recording the activity of its postsynaptic target, the descending contralateral motion detector (DCMD). In each animal, peak DCMD activity occurred a fixed delay ␦ (15 Յ ␦ Յ 35 msec) after the approaching object had reached a specific angular threshold thres on the retina (15°Յ thres Յ 40°). thres was independent of the size or velocity of the approaching object. This angular threshold computation was quite accurate: the error of LGMD and DCMD in estimating thres (3.1-11.9°) corresponds to the angular separation between two and six ommatidia at each edge of the expanding object on the locust retina. It was also resistant to large amplitude changes in background luminosity, contrast, and body temperature. Using several experimentally derived assumptions, the firing rate of LGMD and DCMD could be shown to depend on the product (t Ϫ ␦) ⅐ e Ϫ␣(tϪ␦) , where (t) is the angular size subtended by the object during approach, (t) is the angular edge velocity of the object and the constant, and ␣ is related to the angular threshold size [␣ ϭ 1/tan( thres /2)]. Because LGMD appears to receive distinct input projections, respectively motion-and size-sensitive, this result suggests that a multiplication operation is implemented by LGMD. Thus, LGMD might be an ideal model to investigate the biophysical implementation of a multiplication operation by single neurons.

Sensitivity of excitatory LGMD input to the speed of object motion. Figure S1 below illustrates an experiment that demonstrates the sensitivity of local excitatory input to theLGMD to the speed of object motion. In these experiments, a small black disk (see Krapp and Hengstenberg, Vis. Res., 1997, 37:225-234 for details) was moved at various speeds

Krapp, Holger G., Bärbel Hengstenberg, and Roland Hengstenberg. Dendritic structure and receptive-field organization of optic flow processing interneurons in the fly. J. Neurophysiol. 79: 1902–1917, 1998. The third visual neuropil (lobula plate) of the blowfly Calliphora erythrocephala is a center for processing motion information. It contains, among others, 10 individually identifiable “vertical system” (VS) neurons responding to visual wide-field motions of arbitrary patterns. We demonstrate that each VS neuron is tuned to sense a particular aspect of optic flow that is generated during self-motion. Thus the VS neurons in the fly supply visual information for the control of head orientation, body posture, and flight steering. To reveal the functional organization of the receptive fields of the 10 VS neurons, we determined with a new method the distributions of local motion sensitivities and local preferred directions at 52 positions in the fly's visual field. Each neuron was identified by intracellular staining with Lucifer yellow and three-dimensional reconstructions from 10-μm serial sections. Thereby the receptive-field organization of each recorded neuron could be correlated with the location and extent of its dendritic arborization in the retinotopically organized neuropil of the lobula plate. The response fields of the VS neurons, i.e., the distributions of local preferred directions and local motion sensitivities, are not uniform but resemble rotatory optic flow fields that would be induced by the fly during rotations around various horizontal axes. Theoretical considerations and quantitative analyses of the data, which will be presented in a subsequent paper, show that VS neurons are highly specialized neural filters for optic flow processing and thus for the visual sensation of self-motions in the fly.