THEOBALD, J.C.; O’CARROLL, D.C.; University of Washington; Adelaide University: Spatiotemporal aliasing: a motion illusion in nocturnal hovering hawkmoths

We have found a curious motion illusion in the wide-field motion-sensitive lobula plate cells of certain nocturnal hawkmoths. These cells, found in many insects, are sensitive to spatial and temporal frequency, contrast, and direction of a stimulus. However in nocturnal hovering moths, high temporal frequencies induce a response reversal, as if the stimulus had reversed direction. We term this ‘spatiotemporal aliasing,’ after the well-described phenomena of spatial and temporal aliasing. Additionally, responses followed a pronounced latency at motion onset, but halted after a short exponential decay at offset. Our hypothesis is that these observations are consistent with an unusual delay mechanism in motion detection, which adds a ‘fixed’ latency to the phase-shift of a low-pass filter. Such a delay produces photon summation useful to nocturnal vision without sacrificing the high spatial acuity necessary for hovering. At high temporal frequencies, the discrete spatial sampling of photoreceptors and a fixed temporal delay correlate consecutive cycles of a pattern, creating the illusion of extra motion (hence the name ‘spatiotemporal aliasing’). Further, a correlator type motion detector responds to motion onset only after the signal has passed through both the undelayed and delayed arms, but halts at motion offset when the signal is through just the undelayed arm. So a long fixed delay accounts for the asymmetry between response onset and offset as well.