Computation of object approach by a wide-field, motion-sensitive neuron

The lobula giant motion detector (LGMD) in the locust visual system is a wi de-field, motion-sensitive neuron that responds vigorously to objects appro aching the animal on a collision course. We investigated the computation pe rformed by LGMD when it responds to approaching objects by recording the ac tivity of its postsynaptic target, the descending contralateral motion dete ctor (DCMD). in each animal, peak DCMD activity occurred a fixed delay delt a (15 less than or equal to delta less than or equal to 35 msec) after the approaching object had reached a specific angular threshold theta(thres) on the retina (15 degrees less than or equal to theta(thres) less than or equ al to 40 degrees). theta(thres) was independent of the size or velocity of the approaching object. This angular threshold computation was quite accura te: the error of LGMD and DCMD in estimating theta(thres) (3.1-11.9 degrees ) corresponds to the angular separation between two and six ommatidia at ea ch edge of the expanding object on the locust retina. It was also resistant to large amplitude changes in background luminosity, contrast, and body te mperature. Using several experimentally derived assumptions, the firing rat e of LGMD and DCMD could be shown to depend on the product psi(t - delta).e (-alpha theta(t-delta)), where theta(t) is the angular size subtended by th e object during approach, psi(t) is the angular edge velocity of the object and the constant, and alpha is related to the angular threshold size [alph a = 1/tan(theta(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 multipli cation operation by single neurons.