Motion After-Effects Induced by Dynamic Illumination in Crab Vision.

Motion detection is an elementary aspect of most animal visual systems. However, many environments are prone to background motion, which might disrupt the ability of visual systems to detect relevant motion cues. While in humans, background motion can disrupt the detection of visual cues even after the moving background component has ceased, it remains unknown whether natural forms of background motion might also affect other animal visual systems. Here, we test whether prior exposure to naturally occurring 'caustics', a form of dynamically moving light patterns commonly found in shallow aquatic environments, can have a persisting effect on an animal's motion detection abilities even after the caustic exposure has stopped. To do this, we established the response probability of the shore crab Carcinus maenas to computer-generated expanding disc stimuli mimicking an approaching predator after exposure to either static or moving caustic scenes. Prior exposure to moving caustics had a short-term persisting effect on visual perception in C. maenas, reducing crabs' likelihood to respond to an approaching predator for at least 2 s after the moving caustics had ceased. Our study shows that even after an exposure period to background motion has ended, the visual response rates in C. maenas can still be reduced for a short period owing to the prior exposure. While this so-called 'historical effect' may derive from an adaptation of the crab's visual system to the caustic background motion, we discuss whether it may have survival consequences for this crustacean species.