THEOBALD, J.C.; WARRANT, E.J.; O’CARROLL, D.: Sphingid moths have evolved visual neural filtering adapted to the light levels of their active states

The sphingid moths comprise diurnal, crepuscular, and nocturnal species which fly with remarkable agility: quickly between flowers and hovering while feeding. Despite the drastic differences in luminance during these similar behaviors, sphingids share a common optical eye design (refracting superposition). Because photons are absorbed discretely, moths active in dimmer light deal with increased photon noise, a statistical sampling error in vision that reduces signal to noise ratio. Since optical differences are small, neural strategies must help explain how different moths fly under such different light intensities. Computer models of the motion detection pathway subjected to photon noise show that spatial or temporal low-pass filtering may restore a reliable signal, but at the cost of spatial and temporal acuity. We predict that moths active in dimmer light are forced to sacrifice acuity in order to regain usable motion information. This produces a downward shift in peak spatial and temporal frequency responses and improved signal to noise ratio. We measured spatial and temporal sensitivity of wide-field motion detecting neurons in the brains of diurnal, crepuscular and nocturnal sphingids. Our results support the hypothesis that sphingids use neural filtering to a degree that corresponds to the luminance of their active state environments. Wide-field motion sensitive cells in species active in darker environments have lower spatial and especially lower temporal frequency tuning. We also found a characteristic onset latency of responses in dark-active sphingids. While this presumably slows motion perception through this neural channel, it is consistent with temporal low-pass filtering.