Optic flow induces spatial filtering in fruit flies.
The discrete and random nature of light absorption dictates that, when photons are scarce, images suffer from low signal-to-noise ratios [1]. Nocturnal animals combat this by increasing photon catch, either optically, with large pupils, photoreceptors, and ever larger eyes [2], or neurally, with summation strategies, which sacrifice acuity and leave images blurry and slow [3]. Fast image motion produces the same problems as dim light: photon noise and low signal-to-noise ratio. As a result, fast animals often exhibit optical adaptations comparable to nocturnal animals, but pronounced in lateral regions of the eye where images move quickly [4]. For example, flying insects typically increase equatorial curvature between the front and side of the eye, which increases interommatidial angle and improves light capture. Drosophila, curiously, show only minor changes in interommatidial angle [5]; but even without optical adaptations, they could potentially achieve a similar effect with neural summation. The optimal strategy would manifest as acuity loss, specifically in regions of fast motion, and only parallel to motion. Photoreceptors need quick temporal dynamics to cope with fast motion [6], so the ideal filtering would additionally be spatial. To determine if fruit flies implement filtering driven by background optic flow, I tested their frequency-dependent steering and found that flow transiently eliminates high spatial frequency responses. This effect increases from forward to lateral visual regions, acts only parallel to flow direction, and filters only high spatial, not temporal, responses. Facultative summation may maximize visual information by improving sensitivity during fast flight, but limiting acuity loss when the fly is still.
Department of Biological Sciences, Florida International University, Miami, FL, USA. Electronic address: theobald@fiu.edu.
Jamie
J
Theobald
D000818:Animals; D004331:Drosophila melanogaster / Q000502:physiology*; D005426:Flight, Animal; D057849:Optic Flow / Q000502:physiology*; D000072458:Orientation, Spatial / Q000502:physiology*