Avoiding eye movements: aligning eyes and environment (#395)
In contrast to vertebrates, the eyes of many arthropods are fixed to their heads, slowing down the animal’s ability to rapidly change gaze direction. However, the compound eyes of invertebrates, offer one distinct and unique advantage over the single lens eyes found in vertebrates: there is immense adaptive flexibility in the shape and size of their visual fields. As a consequence, many invertebrates have evolved eyes with large visual fields and heterogeneous properties across the eye. Rather than produce sophisticated scanning eye movements, the animals carefully align such eyes to the topography of the environment. The solution is to look everywhere all the time.
Like the eyes of many crabs, each fiddler crab eye has a full 360 degree visual field that is carefully aligned with the predictable layout of their mostly flat environment. While such wide field vision necessarily leads to a relatively low visual acuity - reducing spatial detail - it removes the need for eye movements and allows different regions of the eye to be adapted to different environmental and behavioural demands. In fiddler crabs, we have found three distinct areas that directly link to the animal’s behavioural demands. While often not as pronounced, such adaptations can also be found in vertebrates, such as fishes and wallabies for example.
Even crabs, however, haven’t given up scanning eye movements altogether. Like flies, spiders and humans they appear to use small and fast eye oscillations to improve their low visual acuity. Both these tactics - panoramic visual fields and micro scanning movements – offer interesting advantages for artificial sensors and have been successfully applied to solve machine vision problems.