A glimpse into crabworld

Almost all known arthropod compound eyes exhibit regional variations of resolving power, absolute light, spectral and polarisation sensitivity which are likely to be matched to the probability of significant events and the availability of cues in the visual world. To understand the signal processing requirements that have led to the evolution of matched sensory and neural filters, we thus need a detailed description of the input signals to a visual system and of the tasks to be performed under natural operating conditions. We report here on the first steps we took in an attempt to reconstruct an animal's specific visual world with emphasis on the motion domain. Fiddler crabs (genus Uca) live in burrows on sand- and mudflats and are active during low tide. They carry their eyes on long, vertically oriented stalks and use vision to detect predators and conspecific signals generated by males waving one massively enlarged claw. The crabs sit on the ground plane of a flat world, where significant events are most likely to occur in a narrow band around the horizon. We recorded scenes in a crab colony with a video camera at crab eye height. The salience of relevant features in the spatial, spectral and polarisation domains was analysed in digitised video images and short sequences of film were processed by a two-dimensional network of motion detectors at various spatial scales. The output of the network provides us with histograms of the direction and strength of motion signals in various spatio-temporal frequency bands. We discuss our results in terms of detection problems, predictability of events, global vs local information content and higher level motion processing involved in intraspecific communication.