Visual Acuity in the Flying Snake, Chrysopelea paradisi

Abstract

Synopsis Visual control during high-speed aerial locomotion requires a visual system adapted for such behaviors. Flying snakes (genus: Chrysopelea) are capable of gliding at speeds up to 11 m s 1 and perform visual assessments before takeoff. Investigating mechanisms of visual control requires a closed-loop experimental system, such as immersive virtual arenas. To characterize vision in the flying snake Chrysopelea paradisi, we used digitally reconstructed models of the head to determine a 3D field of vision. We also used optokinetic drum experiments and compared slow-phase optokinetic nystagmus (OKN) speeds to calculate visual acuity, and conducted preliminary experiments to determine whether snakes would respond to closed-loop virtual stimuli. Visual characterization showed that C. paradisi likely has a large field of view (308.5 6 6.5° azimuthal range), with a considerable binocular region (33.0 6 11.0° azimuthal width) that extends overhead. Their visual systems are broadly tuned and motion-sensitive, with mean peak OKN response gains of 0.50 6 0.11, seen at 46.06 6 11.08 Hz, and a low spatial acuity, with mean peak gain of 0.92 6 0.41, seen at 2.89 6 0.16 cycles per degree (cpd). These characteristics were used to inform settings in an immersive virtual arena, including frame rate, brightness, and stimulus size. In turn, the immersive virtual arena was used to reproduce the optokinetic drum experiments. We elicited OKN in open-loop experiments, with a mean gain of 0.21 6 0.9, seen at 0.019 ± 6 × 105 cpd and 1.79 ± 0.01 Hz. In closed-loop experiments, snakes did not exhibit OKN, but held the image fixed, indicating visual stabilization. These results demonstrate that C. paradisi responds to visual stimuli in a digital virtual arena. The accessibility and adaptability of the virtual setup make it suitable for future studies of visual control in snakes and other animals in an unconstrained setting.