A new function for polarization vision

: mitigation of visual noise from dynamic illumination

Abstract

Caustics are a visual hindrance for animals in shallow underwater environments. Downwelling light is refracted from surface waves onto the seafloor, creating bands of light that fluctuate in both time and space. This dynamic illumination creates a flickering environment that could cause problems for vision, for instance; preventing detection of prey, conspecifics or approaching predators. Maximov (2000) suggested that colour vision may have evolved to mitigate the impact of this unfavourable environmental condition. He suggests that the presence of multiple types of colour receptors and the antagonistic relationship between them could enable discrimination between an object and its background (based on spectral qualities), while ignoring the achromatic variations in illumination caused by caustics. If colour sensitivity was a result of living in habitats subject to water caustics, then polarization sensitivity may have evolved for a similar function. To explore this possibility, crabs (Carcinus maenas) and cuttlefish (Sepia officinalis) were presented with predator-like stimuli (an expanding disc on a screen) overlaid with video of either static or dynamic caustics. Dynamic caustics prevented stimulus detection and greatly reduced response probability, whereas static caustics did not. However, when the stimulus was presented with only a contrast in polarization (instead of in intensity), the ability to detect the expanding disc in dynamic caustics was greatly enhanced, resulting in a similar response probability in both caustic treatments. This study is the first to demonstrate that polarization vision reduces the negative impact of dynamic illumination underwater for visual tasks such as predator-detection.

Date of Award	12 May 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Martin J How (Supervisor) & Tim Caro (Supervisor)