Polarization distance: a framework for modelling object detection by polarization vision systems

Abstract

The discrimination of polarized light is widespread in the natural world. Its use for specific, large-field tasks, such as navigation and the detection of water bodies, has been well documented. Some species of cephalopod and crustacean have polarization receptors distributed across the whole visual field and are thought to use polarized light cues for object detection. Both object-based polarization vision systems and large field detectors rely, at least initially, on an orthogonal, two-channel receptor organization. This may increase to three-directional analysis at subsequent interneuronal levels. In object-based and some of the large-field tasks, the dominant e-vector detection axes are often aligned (through eye, head and body stabilization mechanisms) horizontally and vertically relative to the outside world. We develop Bernard and Wehner's 1977 model of polarization receptor dynamics to apply it to the detection and discrimination of polarized objects against differently polarized backgrounds. We propose a measure of 'polarization distance' (roughly analogous to 'colour distance') for estimating the discriminability of objects in polarized light, and conclude that horizontal/vertical arrays are optimally designed for detecting differences in the degree, and not the e-vector axis, of polarized light under natural conditions.

Null point of discrimination in crustacean polarisation vision

“…This suggests that the horizontal/vertical arrangement of the polarisation receptors is optimally adapted for discriminating small differences in the degree of polarised light (caused, for example, by a conspecific crab) against the horizontally polarised mudflat background (Zeil and Hofmann, 2001;How and Marshall, 2014). However, there is another related explanation for this kind of arrangement.…”

“…The horizontal/vertical receptor organisation of the fiddler crab could therefore be adapted for detecting small changes in the degree of polarisation in this region. This could be used for enhancing the contrast of conspecific crabs against the mudflat surface, and could therefore be thought of as a 'matched filter' for this environment (Wehner, 1987;Zeil and Hofmann, 2001;Alkaladi et al, 2013;How and Marshall, 2014). However, there are several sources of non-horizontally polarised cues.…”

“…One of the main polarisation cues in the fiddler crab's mudflat environment is likely to be found in the area of damp mudflat in the direction of the sun. Here, most reflected light will be roughly horizontally polarised (Zeil and Hofmann, 2001;How and Marshall, 2014). The horizontal/vertical receptor organisation of the fiddler crab could therefore be adapted for detecting small changes in the degree of polarisation in this region.…”

“…Briefly, the model assumes that the animal uses a two-channel polarisation vision system with receptors organised horizontally and vertically relative to the outside world. These receptors then have opponent connections to a simulated visual interneuron, and the relative levels of activity between interneurons viewing the stimulus and the background are compared to produce a single estimate of contrast (see How and Marshall, 2014). For this specific case, we used a polarisation sensitivity (S p ) of 10 for the model.…”

“…Previous studies using this modelling approach identified the problem of potential ambiguities and null points in two-channel polarisation vision systems (Bernard and Wehner, 1977;How and Marshall, 2014). For example, polarised light cues with e-vector axes oriented at +45 or −45 deg stimulate horizontal and vertical polarisation receptors equally, and so are indistinguishable from each other and from unpolarised light of the same intensity.…”