Bilocal visual noise as a probe of wide field motion computation

Using an apparent visual motion stimulus with motion energies limited to specific separations in space and time, we study the computational structure of wide-field motion sensitive neurons in the fly visual brain. There is ample experimental evidence for correlation-based motion computation in many biological systems, but one of its central properties, namely that the response is proportional to the product of two bilocal signal amplitudes, remains to be tested. The design of the apparent motion stimuli used here allows us to manipulate the amplitudes of the bilocal input signals that serve as inputs to the computation. We demonstrate that the wide-field motion response of H1 and V1 neurons indeed shows bilinear behavior, even under contrast sign reversal, as predicted. But the response also varies inversely with contrast variance, an effect not described by the correlator operation. We also quantify the correlator contributions for different spatial and temporal separations. With suitable modification, the apparent motion stimuli used here can be applied to a broad range of neurophysiological as well as human psychophysical studies on motion perception.